Xen: Fix snapshots when use_cow=True.
Xen: Fix snapshots when use_cow=True.

This fixes an issue where use_cow=True would sporadically cause snapshot
corruption.

The root cause was a race-condition where the VM's sm_config was
sometimes out-of-sync with the underlying VHDs.

The solution is to force an SR rescan immediately after taking a
snapshot so that any VHD parent-pointer modifications are propogated
into the sm_config so that _walk_vdi_chain will receive accurate
information.

Change-Id: I03b4914d9420918590ae27dfc77dfedac9c767c1

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