core: keep locks for whole offline disk migration process

The VM lock is now kept for the whole offline (not live) disk migration
process. This change prevents a race that can cause a VM to start
running in parallel to offline disk migration.

Change-Id: I89acf9f66bce647a7ecb2dd407e51a1ba03f4e17
Bug-Url: https://bugzilla.redhat.com/952147
Signed-off-by: Arik Hadas <ahadas@redhat.com>