qemu: Set QEMU_AUDIO_DRV=none with -nographic

On my machine, a guest fails to boot if it has a sound card, but not
graphical device/display is configured, because pulseaudio fails to
initialize since it can't access $HOME.

A workaround is removing the audio device, however on ARM boards there
isn't any option to do that, so -nographic always fails.

Set QEMU_AUDIO_DRV=none if no <graphics> are configured. Unfortunately
this has massive test suite fallout.

Add a qemu.conf parameter nographics_allow_host_audio, that if enabled
will pass through QEMU_AUDIO_DRV from sysconfig (similar to
vnc_allow_host_audio)

tests/qemuxml2argvdata/qemuxml2argv-cpu-host-model-fallback.args

@@ -2,7 +2,7 @@ LC_ALL=C \
 PATH=/bin \
 HOME=/home/test \
 USER=test \
-LOGNAME=test \
+LOGNAME=test QEMU_AUDIO_DRV=none \
 /usr/bin/qemu \
 -S \
 -M pc \