ui/vnc: Require audiodev= to enable audio

If there is no audiodev do not send the audio ack in response to
VNC_ENCODING_AUDIO, so that clients aren't told audio exists, and
immediately drop the client if they try to send any audio control messages
when audio is not advertised.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>