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>

audio/audio_int.h

@@ -31,6 +31,10 @@
 #endif
 #include "mixeng.h"
 
+#ifdef CONFIG_GIO
+#include <gio/gio.h>
+#endif
+
 struct audio_pcm_ops;
 
 struct audio_callback {
@@ -140,6 +144,9 @@ struct audio_driver {
     const char *descr;
     void *(*init) (Audiodev *);
     void (*fini) (void *);
+#ifdef CONFIG_GIO
+    void (*set_dbus_server) (AudioState *s, GDBusObjectManagerServer *manager);
+#endif
     struct audio_pcm_ops *pcm_ops;
     int can_be_default;
     int max_voices_out;