core: live migration for disks based on template

Fetch DiskImage using get() instead of getAncestor()
(getAncestor() returns template's disk when applicable)

Change-Id: I42747d9d5d7f55e457c4b2f9d05b1c7b02dffb3d
Bug-Url: https://bugzilla.redhat.com/896439
Signed-off-by: Daniel Erez <derez@redhat.com>