libqtest: launch QEMU with QEMU_AUDIO_DRV=none

No test case actually uses the audio backend.  Disable audio to prevent
warnings on hosts with no sound hardware present:

  GTESTER check-qtest-aarch64
  sdl: SDL_OpenAudio failed
  sdl: Reason: No available audio device
  sdl: SDL_OpenAudio failed
  sdl: Reason: No available audio device
  audio: Failed to create voice `lm4549.out'
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NGerd Hoffmann <kraxel@redhat.com>

tests/libqtest.c

@@ -165,6 +165,7 @@ QTestState *qtest_init(const char *extra_args)
 
     s->qemu_pid = fork();
     if (s->qemu_pid == 0) {
+        setenv("QEMU_AUDIO_DRV", "none", true);
         command = g_strdup_printf("exec %s "
                                   "-qtest unix:%s,nowait "
                                   "-qtest-log %s "