qemu: remove default audio backend for migratable XML

qemu: remove default audio backend for migratable XML

When seeing a guest with a sound device, and no audio backend, we
automatically add an audio backend XML element based on the historical
QEMU driver behaviour. Unfortunately when we live migrate back to an
old libvirt, it may not understand the audio driver type we configured.
We thus need to strip the default audio backend when migrating.

Fixes https://gitlab.com/libvirt/libvirt/-/issues/179
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>