audio/pa: convert to QOM lifecycle methods

Migrate the PulseAudio backend from the legacy driver init/fini
callbacks to proper QOM realize and finalize methods.

The paaudio struct fields are now embedded directly in the AudioPa
QOM object instead of being allocated separately as drv_opaque. This
allows accessing the backend state through proper QOM type casting
with AUDIO_PA() rather than casting drv_opaque pointers.

The PulseAudio connection is now managed through the QOM lifecycle,
with the connection reference acquired in realize and released in
finalize.

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>