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..