Fix backing file cp/resize race condition.
Fix backing file cp/resize race condition.

 * Fixes bug 953831

Change-Id: I39950b16c9b76159b16203f7b8b504cae5475516

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