io: tls: Add qio_channel_tls_bye

io: tls: Add qio_channel_tls_bye

Add a task dispatcher for gnutls_bye similar to the
qio_channel_tls_handshake_task(). The gnutls_bye() call might be
interrupted and so it needs to be rescheduled.

The migration code will make use of this to help the migration
destination identify a premature EOF. Once the session termination is
in place, any EOF that happens before the source issued gnutls_bye()
will be considered an error.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>