hw/ac97: add support for volume control

Combine output volume with Master and PCM registers values.
Use default values in mixer_reset ().
Set volume on post-load to update backend values.

v4,v5:
- fix some code style

Signed-off-by: Marc-Andr? Lureau <marcandre.lureau@redhat.com>
Signed-off-by: malc <av1474@comtv.ru>