migration/postcopy: mis->have_listen_thread check will never be touched

migration/postcopy: mis->have_listen_thread check will never be touched

If mis->have_listen_thread is true, this means current PostcopyState
must be LISTENING or RUNNING. While the check at the beginning of the
function makes sure the state transaction happens when its previous
PostcopyState is ADVISE or DISCARD.

This means we would never touch this check.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20191006000249.29926-2-richardw.yang@linux.intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>