audio: Never send migration section

The audio migration vmstate is empty, and always has been; we can't
just remove it though because an old qemu might send it us.
Changes with -audiodev now mean it's sometimes created when it didn't
used to be, and can confuse migration to old qemu.

Change it so that vmstate_audio is never sent; if it's received it
should still be accepted, and old qemu's shouldn't be too upset if it's
missing.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210809170956.78536-1-dgilbert@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

audio/audio.c

@@ -1622,10 +1622,20 @@ void audio_cleanup(void)
     }
 }
 
+static bool vmstate_audio_needed(void *opaque)
+{
+    /*
+     * Never needed, this vmstate only exists in case
+     * an old qemu sends it to us.
+     */
+    return false;
+}
+
 static const VMStateDescription vmstate_audio = {
     .name = "audio",
     .version_id = 1,
     .minimum_version_id = 1,
+    .needed = vmstate_audio_needed,
     .fields = (VMStateField[]) {
         VMSTATE_END_OF_LIST()
     }