xenapi: Only coalesce VHDs if needed
xenapi: Only coalesce VHDs if needed

If the original parent is None, then we don't need to coalesce the VHD since
there won't be a grandparent for the child to coalesce into. Doing otherwise,
causes us to poll forever as we wait for the child's parent to become `None`.

Fixes bug 1200806

Change-Id: I0cbf0be86ff925b83c0b8fa90f88d4ca304fd2b9

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