audio: add "dbus" audio backend

Add a new -audio backend that accepts D-Bus clients/listeners to handle
playback & recording, to be exported via the -display dbus.

Example usage:
-audiodev dbus,in.mixing-engine=off,out.mixing-engine=off,id=dbus
-display dbus,audiodev=dbus

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>

qapi/audio.json

@@ -386,7 +386,7 @@
 # Since: 4.0
 ##
 { 'enum': 'AudiodevDriver',
-  'data': [ 'none', 'alsa', 'coreaudio', 'dsound', 'jack', 'oss', 'pa',
+  'data': [ 'none', 'alsa', 'coreaudio', 'dbus', 'dsound', 'jack', 'oss', 'pa',
             'sdl', 'spice', 'wav' ] }
 
 ##
@@ -412,6 +412,7 @@
     'none':      'AudiodevGenericOptions',
     'alsa':      'AudiodevAlsaOptions',
     'coreaudio': 'AudiodevCoreaudioOptions',
+    'dbus':      'AudiodevGenericOptions',
     'dsound':    'AudiodevDsoundOptions',
     'jack':      'AudiodevJackOptions',
     'oss':       'AudiodevOssOptions',

qapi/ui.json

@@ -1134,13 +1134,16 @@
 # @p2p: Whether to use peer-to-peer connections (accepted through
 #       ``add_client``).
 #
+# @audiodev: Use the specified DBus audiodev to export audio.
+#
 # Since: 7.0
 #
 ##
 { 'struct'  : 'DisplayDBus',
   'data'    : { '*rendernode' : 'str',
                 '*addr': 'str',
-                '*p2p': 'bool' } }
+                '*p2p': 'bool',
+                '*audiodev': 'str' } }
 
  ##
  # @DisplayGLMode: