qemu: populate <audio> element with default config

Currently the QEMU driver secretly sets the QEMU_AUDIO_DRV env variable

 - VNC - set to "none", unless passthrough of host env variable is set
 - SPICE - always set to "spice"
 - SDL - always passthrough host env
 - No graphics - set to "none", unless passthrough of host env variable is set

The setting of the QEMU_AUDIO_DRV env variable is done in the code which
configures graphics.

If no <audio> element is present, we now auto-populate <audio> elements
to reflect this historical default config. This avoids need to set audio
env when processing graphics.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

tests/qemuhotplugtestdomains/qemuhotplug-base-live+interface-vdpa.xml

@@ -51,6 +51,7 @@
     <input type='keyboard' bus='ps2'>
       <alias name='input1'/>
     </input>
+    <audio id='1' type='none'/>
     <memballoon model='none'/>
   </devices>
   <seclabel type='none' model='none'/>