Add volume management to AVR

demo.py

@@ -79,6 +79,7 @@ class DemoController:
             ],
             "avr": {
                 "connected": True, "inputs": self.avr.inputs(), "input": self.avr.current_input(),
+                "volume": self._volume[self.cfg.HOST_KEY],
                 # Standing in for a Zidoo plugged into this input, so the
                 # AVR card's now-playing block has something to show here too.
                 "now_playing": {"song": "Big Buck Bunny", "artist": "2008", "image": None,