-audio is used like "-audio pa,model=sb16".  It is almost as simple as
-soundhw, but it reuses the -audiodev parsing machinery and attaches an
audiodev to the newly-created device.  The main 'feature' is that
it knows about adding the codec device for model=intel-hda, and adding
the audiodev to the codec device.

In the future, it could be extended to support default models or
builtin devices, just like -nic, or even a default backend.  For now,
keep it simple.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>