add volume synchronization to protocol
add volume synchronization to protocol

With VD_AGENT_AUDIO_VOLUME_SYNC the client can send volume and mute
values to be set in the guest for input or output devices.
Currently this is done once after the agent send its capabilities.

Related: https://bugzilla.redhat.com/show_bug.cgi?id=1012868

SPICE/spice-protocol,Fantu/spice-protocol,vdiot/spice-protocol,fgouget/spice-protocol,SPICE/spice-protocol,vdiot/spice-protocol,Fantu/spice-protocol,fgouget/spice-protocol,SPICE/spice-protocol,vdiot/spice-protocol,fgouget/spice-protocol