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)
qemu allows the user to choose what io storage api should be used,
either the default (threads) or native (linux aio) which in the latter
case can result in better performance.
Based on a patch originally by Matthias Dahl.
Red Hat Bugzilla #591703
Signed-off-by: Eric Blake <eblake@redhat.com>