qemu: allow pulseaudio to be the default

We're seeing various issues with the SDL audio backend and want to
switch to the pulseaudio backend. See e.g.

  https://bugzilla.redhat.com/495964
  https://bugzilla.redhat.com/519540
  https://bugzilla.redhat.com/496627

The pulseaudio backend seems to work well, so we should allow it to be
selected as the default.
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: Nmalc <av1474@comtv.ru>

audio/paaudio.c

@@ -519,7 +519,7 @@ struct audio_driver pa_audio_driver = {
     .init           = qpa_audio_init,
     .fini           = qpa_audio_fini,
     .pcm_ops        = &qpa_pcm_ops,
-    .can_be_default = 0,
+    .can_be_default = 1,
     .max_voices_out = INT_MAX,
     .max_voices_in  = INT_MAX,
     .voice_size_out = sizeof (PAVoiceOut),