virtio-sound: implement audio output (TX)

virtio-sound: implement audio output (TX)

Handle output IO messages in the transmit (TX) virtqueue.

It allocates a VirtIOSoundPCMBuffer for each IO message and copies the
data buffer to it. When the IO buffer is written to the host's sound
card, the guest will be notified that it has been consumed.

The lifetime of an IO message is:

1. Guest sends IO message to TX virtqueue.
2. QEMU adds it to the appropriate stream's IO buffer queue.
3. Sometime later, the host audio backend calls the output callback,
   virtio_snd_pcm_out_cb(), which is defined with an AUD_open_out()
   call. The callback gets an available number of bytes the backend can
   receive. Then it writes data from the IO buffer queue to the backend.
   If at any time a buffer is exhausted, it is returned to the guest as
   completed.
4. If the guest releases the stream, its buffer queue is flushed by
   attempting to write any leftover data to the audio backend and
   releasing all IO messages back to the guest. This is how according to
   the spec the guest knows the release was successful.

Based-on: https://github.com/OpenSynergy/qemu/commit/5a2f350eec5d157b90d9c7b40a8e603f4da92471
Signed-off-by: Igor Skalkin <Igor.Skalkin@opensynergy.com>
Signed-off-by: Anton Yakovlev <Anton.Yakovlev@opensynergy.com>
Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <b7c6fc458c763d09a4abbcb620ae9b220afa5b8f.1698062525.git.manos.pitsidianakis@linaro.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>