core: disk live migrate - snapshot only on migrated disks

LiveSnapshotTaskHandler:
Take a snapshot only on the migrated disks
(by utilizing single disk snapshot feature).

Change-Id: I21387ef38b5f86806b985b8dffb49d5b7952a628
Bug-Url: https://bugzilla.redhat.com/969789
Bug-Url: https://bugzilla.redhat.com/1056922
Signed-off-by: Daniel Erez <derez@redhat.com>