Reintroduce afterMigrationStatus

In http://gerrit.ovirt.org/19306 I was fooled to believe that we can
drop the unused afterMigrationStatus parameter. However vdsm version
prior to that change (e.g. that of ovirt-3.3.0) expect to delete it.

Change-Id: Ibc55f7fc0d7debd1590011301c38565d27dc137e
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19859
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>