ui: convert VNC server to use QIOChannelTLS

Switch VNC server over to using the QIOChannelTLS object for
the TLS session. This removes all remaining VNC specific code
for dealing with TLS handshakes.

Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>