core: update images size in DB after migration

Images actual size should be updated in db after live
disk migration (info data is fetched using GetImageInfo).

Change-Id: I4ec8e2e0c8017b6576f9a849d419e87d6d509c69
Bug-Url: https://bugzilla.redhat.com/878460
Signed-off-by: Daniel Erez <derez@redhat.com>