docs/system: add basic virtio-snd documentation

This commit adds basic documentation for using virtio-snd.

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: <e7fb941cf7636fdff40cbdcdcd660dec5f15ca3c.1698062525.git.manos.pitsidianakis@linaro.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

MAINTAINERS

@@ -2317,6 +2317,7 @@ S: Supported
 F: hw/audio/virtio-snd.c
 F: hw/audio/virtio-snd-pci.c
 F: include/hw/audio/virtio-snd.h
+F: docs/system/devices/virtio-snd.rst
 
 nvme
 M: Keith Busch <kbusch@kernel.org>

docs/system/device-emulation.rst

@@ -93,6 +93,7 @@ Emulated Devices
    devices/vhost-user.rst
    devices/virtio-gpu.rst
    devices/virtio-pmem.rst
+   devices/virtio-snd.rst
    devices/vhost-user-rng.rst
    devices/canokey.rst
    devices/usb-u2f.rst

docs/system/devices/virtio-snd.rst

+virtio sound
+============
+
+This document explains the setup and usage of the Virtio sound device.
+The Virtio sound device is a paravirtualized sound card device.
+
+Linux kernel support
+--------------------
+
+Virtio sound requires a guest Linux kernel built with the
+``CONFIG_SND_VIRTIO`` option.
+
+Description
+-----------
+
+Virtio sound implements capture and playback from inside a guest using the
+configured audio backend of the host machine.
+
+Device properties
+-----------------
+
+The Virtio sound device can be configured with the following properties:
+
+ * ``jacks`` number of physical jacks (Unimplemented).
+ * ``streams`` number of PCM streams. At the moment, no stream configuration is supported: the first one will always be a playback stream, an optional second will always be a capture stream. Adding more will cycle stream directions from playback to capture.
+ * ``chmaps`` number of channel maps (Unimplemented).
+
+All streams are stereo and have the default channel positions ``Front left, right``.
+
+Examples
+--------
+
+Add an audio device and an audio backend at once with ``-audio`` and ``model=virtio``:
+
+ * pulseaudio: ``-audio driver=pa,model=virtio``
+   or ``-audio driver=pa,model=virtio,server=/run/user/1000/pulse/native``
+ * sdl: ``-audio driver=sdl,model=virtio``
+ * coreaudio: ``-audio driver=coreaudio,model=virtio``
+
+etc.
+
+To specifically add virtualized sound devices, you have to specify a PCI device
+and an audio backend listed with ``-audio driver=help`` that works on your host
+machine, e.g.:
+
+::
+
+  -device virtio-sound-pci,audiodev=my_audiodev \
+  -audiodev alsa,id=my_audiodev