audio/sndio: convert to QOM lifecycle methods

Remove the legacy driver init/fini callbacks from the sndio audio backend.

Both sndio_audio_init() and sndio_audio_fini() were no-ops that
performed no real initialization or cleanup work. Access to the
Audiodev is now through hw->s->dev instead of the drv_opaque pointer.

Reviewed-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>