Provide a helper for non-COLO use case of migration to stop a VM.  This
prepares for adding some downtime relevant tracepoints to migration, where
they may or may not apply to COLO.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231030163346.765724-5-peterx@redhat.com>