handle updated qemu-img info output
handle updated qemu-img info output

Originally `qemu-img info` always output an (actual path: ...)
even if it was duplicated with that already on the line.

 $ instances=/var/lib/nova/instances/
 $ qemu-img info $instances/instance-00000017/disk | grep 'backing'
 backing file: $instances/_base/24083... (actual path: $the_same)

Whereas after the change referenced at:
https://lists.gnu.org/archive/html/qemu-devel/2012-05/msg01468.html
It suppresses a duplicate (actual path:)

 $ instances=/var/lib/nova/instances/
 $ qemu-img info $instances/instance-00000017/disk | grep 'backing'
 backing file: $instances/_base/24083...

* nova/virt/libvirt/utils.py (get_disk_backing_file):
Avoid an indexError exception when parsing the newer format.
Fixes bug 1000261

Change-Id: Ie2889b6da8a5c93e0e874e7a330529f6e6e71b0b

belmiromoreira/nova,ewindisch/nova,paulmathews/nova,JianyuWang/nova,angdraug/nova,barnsnake351/nova,paulmathews/nova,akash1808/nova_test_latest,vmturbo/nova,alexandrucoman/vbox-nova-driver,bgxavier/nova,affo/nova,yrobla/nova,gooddata/openstack-nova,JioCloud/nova_test_latest,cloudbase/nova,gooddata/openstack-nova,vmturbo/nova,NeCTAR-RC/nova,iuliat/nova,fnordahl/nova,klmitch/nova,jianghuaw/nova,cloudbase/nova-virtualbox,citrix-openstack-build/nova,j-carpentier/nova,dawnpower/nova,usc-isi/nova,rajalokan/nova,paulmathews/nova,isyippee/nova,cernops/nova,joker946/nova,leilihh/nova,zzicewind/nova,petrutlucian94/nova_dev,Metaswitch/calico-nova,gooddata/openstack-nova,rajalokan/nova,MountainWei/nova,yrobla/nova,hanlind/nova,tangfeixiong/nova,varunarya10/nova_test_latest,rickerc/nova_audit,BeyondTheClouds/nova,spring-week-topos/nova-week,plumgrid/plumgrid-nova,Stavitsky/nova,devendermishrajio/nova,aristanetworks/arista-ovs-nova,shootstar/novatest,savi-dev/nova,tealover/nova,petrutlucian94/nova_dev,angdraug/nova,mgagne/nova,apporc/nova,CiscoSystems/nova,cloudbase/nova,saleemjaveds/https-github.com-openstack-nova,gooddata/openstack-nova,openstack/nova,thomasem/nova,orbitfp7/nova,iuliat/nova,savi-dev/nova,MountainWei/nova,eonpatapon/nova,Tehsmash/nova,Yusuke1987/openstack_template,usc-isi/nova,viggates/nova,usc-isi/extra-specs,eayunstack/nova,maelnor/nova,mahak/nova,DirectXMan12/nova-hacking,dstroppa/openstack-smartos-nova-grizzly,CiscoSystems/nova,Yuriy-Leonov/nova,rahulunair/nova,savi-dev/nova,scripnichenko/nova,akash1808/nova,dawnpower/nova,TieWei/nova,Yuriy-Leonov/nova,takeshineshiro/nova,j-carpentier/nova,gspilio/nova,cernops/nova,aristanetworks/arista-ovs-nova,Francis-Liu/animated-broccoli,gspilio/nova,ted-gould/nova,sebrandon1/nova,virtualopensystems/nova,whitepages/nova,usc-isi/extra-specs,klmitch/nova,whitepages/nova,tianweizhang/nova,dstroppa/openstack-smartos-nova-grizzly,maheshp/novatest,petrutlucian94/nova,jeffrey4l/nova,OpenAcademy-OpenStack/nova-scheduler,zhimin711/nova,openstack/nova,CEG-FYP-OpenStack/scheduler,zzicewind/nova,sridevikoushik31/nova,LoHChina/nova,cernops/nova,mikalstill/nova,sebrandon1/nova,dims/nova,ruslanloman/nova,leilihh/novaha,sridevikoushik31/openstack,hanlind/nova,noironetworks/nova,TieWei/nova,NoBodyCam/TftpPxeBootBareMetal,eayunstack/nova,maoy/zknova,double12gzh/nova,TwinkleChawla/nova,cloudbau/nova,shail2810/nova,devoid/nova,varunarya10/nova_test_latest,vladikr/nova_drafts,cloudbase/nova,eonpatapon/nova,adelina-t/nova,devendermishrajio/nova_test_latest,luogangyi/bcec-nova,jianghuaw/nova,rajalokan/nova,mgagne/nova,rahulunair/nova,leilihh/novaha,apporc/nova,affo/nova,mandeepdhami/nova,DirectXMan12/nova-hacking,JioCloud/nova,phenoxim/nova,fnordahl/nova,CCI-MOC/nova,BeyondTheClouds/nova,rickerc/nova_audit,viggates/nova,zhimin711/nova,dstroppa/openstack-smartos-nova-grizzly,ted-gould/nova,imsplitbit/nova,belmiromoreira/nova,klmitch/nova,Juniper/nova,TwinkleChawla/nova,badock/nova,yatinkumbhare/openstack-nova,CloudServer/nova,houshengbo/nova_vmware_compute_driver,eharney/nova,noironetworks/nova,eharney/nova,Triv90/Nova,maelnor/nova,vladikr/nova_drafts,sebrandon1/nova,kimjaejoong/nova,silenceli/nova,virtualopensystems/nova,projectcalico/calico-nova,projectcalico/calico-nova,double12gzh/nova,qwefi/nova,silenceli/nova,Triv90/Nova,citrix-openstack-build/nova,JianyuWang/nova,rajalokan/nova,Metaswitch/calico-nova,luogangyi/bcec-nova,tealover/nova,Juniper/nova,NewpTone/stacklab-nova,maheshp/novatest,saleemjaveds/https-github.com-openstack-nova,klmitch/nova,yrobla/nova,SUSE-Cloud/nova,isyippee/nova,cyx1231st/nova,NewpTone/stacklab-nova,sacharya/nova,leilihh/nova,devoid/nova,nikesh-mahalka/nova,berrange/nova,sridevikoushik31/nova,plumgrid/plumgrid-nova,devendermishrajio/nova,BeyondTheClouds/nova,mahak/nova,openstack/nova,thomasem/nova,mikalstill/nova,dims/nova,sridevikoushik31/nova,OpenAcademy-OpenStack/nova-scheduler,alexandrucoman/vbox-nova-driver,Juniper/nova,Stavitsky/nova,phenoxim/nova,rrader/nova-docker-plugin,watonyweng/nova,rahulunair/nova,blueboxgroup/nova,yosshy/nova,tudorvio/nova,akash1808/nova,raildo/nova,imsplitbit/nova,maheshp/novatest,mandeepdhami/nova,jianghuaw/nova,josephsuh/extra-specs,maoy/zknova,shootstar/novatest,usc-isi/extra-specs,NewpTone/stacklab-nova,raildo/nova,Tehsmash/nova,NoBodyCam/TftpPxeBootBareMetal,badock/nova,tianweizhang/nova,ruslanloman/nova,ewindisch/nova,devendermishrajio/nova_test_latest,jeffrey4l/nova,vmturbo/nova,qwefi/nova,CCI-MOC/nova,felixma/nova,takeshineshiro/nova,kimjaejoong/nova,watonyweng/nova,sridevikoushik31/openstack,zaina/nova,joker946/nova,mahak/nova,usc-isi/nova,yatinkumbhare/openstack-nova,tanglei528/nova,bgxavier/nova,aristanetworks/arista-ovs-nova,petrutlucian94/nova,bigswitch/nova,mikalstill/nova,JioCloud/nova,tangfeixiong/nova,alvarolopez/nova,alvarolopez/nova,LoHChina/nova,shahar-stratoscale/nova,edulramirez/nova,vmturbo/nova,ntt-sic/nova,houshengbo/nova_vmware_compute_driver,bigswitch/nova,josephsuh/extra-specs,edulramirez/nova,tanglei528/nova,akash1808/nova_test_latest,sridevikoushik31/openstack,Yusuke1987/openstack_template,shail2810/nova,cyx1231st/nova,spring-week-topos/nova-week,sridevikoushik31/nova,hanlind/nova,NoBodyCam/TftpPxeBootBareMetal,Juniper/nova,maoy/zknova,sacharya/nova,scripnichenko/nova,bclau/nova,nikesh-mahalka/nova,fajoy/nova,redhat-openstack/nova,felixma/nova,alaski/nova,Triv90/Nova,bclau/nova,SUSE-Cloud/nova,NeCTAR-RC/nova,yosshy/nova,alaski/nova,jianghuaw/nova,Francis-Liu/animated-broccoli,houshengbo/nova_vmware_compute_driver,CloudServer/nova,josephsuh/extra-specs,Brocade-OpenSource/OpenStack-DNRM-Nova,berrange/nova,barnsnake351/nova,cloudbau/nova,zaina/nova,cloudbase/nova-virtualbox,tudorvio/nova,redhat-openstack/nova,orbitfp7/nova,ntt-sic/nova,fajoy/nova,CEG-FYP-OpenStack/scheduler,shahar-stratoscale/nova,gspilio/nova,rrader/nova-docker-plugin,fajoy/nova,DirectXMan12/nova-hacking,JioCloud/nova_test_latest,adelina-t/nova,mmnelemane/nova,Brocade-OpenSource/OpenStack-DNRM-Nova,blueboxgroup/nova,mmnelemane/nova