core: LSM - lock image on CreateImagePlaceholder phase

Lock image on CreateImagePlaceholder phase
rather than on LiveSnapshot endSuccessfully().
(prevents locked disks when shutting down
the VM during live snapshot).

Change-Id: I899e55c995a96f68023e2ad7b31daac57d1e8dbb
Bug-Url: https://bugzilla.redhat.com/896445
Signed-off-by: Daniel Erez <derez@redhat.com>