libvirt: Fixes live migration for volume backed instances

Live migration fails for volume backed instances in LibvirtDriver
because _is_shared_block_storage() incorrectly identifies volume
backed disks as local. This is fixed by passing the block_device_info
parameter to get_instance_disk_info, which allows for correct
filtering of volume backed disks.

Change-Id: I1437b2a7d5a62615b0099114ed1b5b1110f56de2
Closes-bug: 1392773
Merge "libvirt: Fixes live migration for volume backed instances"