protocol: add SPICE_{PLAYBACK,RECORD}_CAP_VOLUME caps
protocol: add SPICE_{PLAYBACK,RECORD}_CAP_VOLUME caps

These changes the protocol the following way:

When the client announces it has SPICE_PLAYBACK_CAP_VOLUME, the server
can send PlaybackChannel.{volume,mute} messages to notify of volume
changes (respectively for RECORD).

If the client doesn't implement SPICE_PLAYBACK_CAP_VOLUME, the server can
handle the volume changes the way it prefers, for example, ignoring them or
applying volume effect on the server side etc..

vdiot/spice-protocol,elmarco/spice-protocol,fgouget/spice-protocol,elmarco/spice-protocol,dezelin/spice-protocol,vdiot/spice-protocol,SPICE/spice-protocol,dezelin/spice-protocol,freedesktop-unofficial-mirror/spice__spice-protocol,Fantu/spice-protocol,Fantu/spice-protocol,elmarco/spice-protocol,fgouget/spice-protocol,vdiot/spice-protocol,dezelin/spice-protocol,SPICE/spice-protocol,fgouget/spice-protocol,SPICE/spice-protocol,freedesktop-unofficial-mirror/spice__spice-protocol