Make qio_channel_tls_shutdown thread-safe by using atomics when
accessing tioc->shutdown.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <5bd8733f583f3558b32250fd0eb576b7aa756485.1609167865.git.lukasstraub2@web.de>
Signed-off-by: Markus Armbruster <armbru@redhat.com>