libvirt: support live migrations of instances with config drives

In case of shared storage, to allow an instance with a configdrive
to be migrated if we must store the configdrive into the same backend
as other disks.

Related to bug #1246201

Change-Id: I699f0746da03f7ef669d68665996c839d4d99ee4
Merge "libvirt: support live migrations of instances with config drives"