core: lock VM for the whole VM migration process

Exclusive VM lock is now being taken for the whole execution of VM
migration, thus preventing other operations that should not be run in
parallel from being executed (as long as they take VM lock, which can be
shared lock, as well of course).

Change-Id: Iec678dfda4824e9ece18f0c876ffc94594591ff8
Bug-Url: https://bugzilla.redhat.com/952147
Signed-off-by: Arik Hadas <ahadas@redhat.com>