audio: allow returning an error from the driver init

An error is already printed by audio_driver_init, but we can make
it more precise if the driver can return an Error *.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/audio/wavaudio.c b/audio/wavaudio.c
index 6445a2c..26b0390 100644
--- a/audio/wavaudio.c
+++ b/audio/wavaudio.c
@@ -182,7 +182,7 @@
     }
 }
 
-static void *wav_audio_init(Audiodev *dev)
+static void *wav_audio_init(Audiodev *dev, Error **errp)
 {
     assert(dev->driver == AUDIODEV_DRIVER_WAV);
     return dev;