add audio support to vnc

Hi,

I believe there is a reason for this to not be supported yet but to get the discussion/reason going, I did a simple MR that would give audio over VNC on remote-viewer.

If this seems reasonable to have, should it go to gtk-vnc instead? (spice-gtk does handle audio backend and only exposes some API for enable/disable/..). Pretty sure that at least a way to disable the audio would be needed, command line option is enough?

Cheers,