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: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
