core: unlock the VM after unsuccessful migration

This patch fixes a regression that caused VM to remain locked after
migration fails (in all rerun attempts).

Change-Id: I83de3934f801bf5c9be22173b5caf8ae0bf5b519
Bug-Url: https://bugzilla.redhat.com/1086280
Signed-off-by: Arik Hadas <ahadas@redhat.com>