Libvirt: disallow live-mig for volume-backed with local disk

This patch makes libvirt raise an error if a live migration was
requested without shared storage for a volume-backed instance, if that
instance has any local disks.

The reason is that without shared storage, local disks will be
re-created on the destination node which can result in loss of data.

Change-Id: Ic96dabf6020e957309280862b325792faf44b1f5
Closes-bug: 1236356
Merge "Libvirt: disallow live-mig for volume-backed with local disk"