qemu: Allow audio driver override in virtqemud

libvirtd supports this feature, and virtqemud ultimately calls to
the same code so it does as well: advertise it in the sysconf file
for the latter, as is already the case for the former.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/qemu/virtqemud.sysconf

 # Customizations for the virtqemud.service systemd unit
 
 VIRTQEMUD_ARGS="--timeout 120"
+
+# Override the QEMU/SDL default audio driver probing when
+# starting virtual machines using SDL graphics
+#
+# NB these have no effect for VMs using VNC, unless vnc_allow_host_audio
+# is enabled in /etc/libvirt/qemu.conf
+#QEMU_AUDIO_DRV=sdl
+#
+#SDL_AUDIODRIVER=pulse