audio: don't build alsa and sdl by default on linux

In case no sound hardware is present both alsa and sdl drivers
initialize successfully and throw errors later on, i.e. effectively
the automatic probing doesn't work.  Drop them from the list of
default audio drivers for linux because of that.

Fixes: 6a485418 audio: probe audio drivers by default
Buglink: https://bugs.launchpad.net/qemu/+bug/1816052


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: David Hildenbrand <david@redhat.com>
Message-id: 20190219124257.3001-1-kraxel@redhat.com