Wrapper for QMP command "migrate-pause".

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180502104740.12123-25-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>