conf: introduce support for common audio settings

conf: introduce support for common audio settings

This introduces support for the QEMU audio settings that are common to
all audio backends. These are expressed in the QAPI schema as settings
common to all backends, but in reality some backends ignore some of
them. For example, some backends are output only. The parser isn't
attempting to apply restrictions that QEMU itself doesn't apply.

    <audio id='1' type='pulseaudio'>
      <input mixingEngine='yes' fixedSettings='yes' voices='1' bufferLength='100'>
        <settings frequency='44100' channels='2' format='s16'/>
      </input>
      <output mixingEngine='yes' fixedSettings='yes' voices='2' bufferLength='100'>
        <settings frequency='22050' channels='4' format='f32'/>
      </output>
    </audio>

The <settings> child is only valid if fixedSettings='yes'

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