docs: Add documentation for vhost based RNG implementation

Add description and example for the vhost-user based RNG implementation.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Message-Id: <20211012205904.4106769-4-mathieu.poirier@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/system/device-emulation.rst

@@ -88,3 +88,4 @@ Emulated Devices
    devices/usb.rst
    devices/vhost-user.rst
    devices/virtio-pmem.rst
+   devices/vhost-user-rng.rst

docs/system/devices/vhost-user-rng.rst

@@ -0,0 +1,39 @@
+QEMU vhost-user-rng - RNG emulation
+===================================
+
+Background
+----------
+
+What follows builds on the material presented in vhost-user.rst - it should
+be reviewed before moving forward with the content in this file.
+
+Description
+-----------
+
+The vhost-user-rng device implementation was designed to work with a random
+number generator daemon such as the one found in the vhost-device crate of
+the rust-vmm project available on github [1].
+
+[1]. https://github.com/rust-vmm/vhost-device
+
+Examples
+--------
+
+The daemon should be started first:
+
+::
+
+  host# vhost-device-rng --socket-path=rng.sock -c 1 -m 512 -p 1000
+
+The QEMU invocation needs to create a chardev socket the device can
+use to communicate as well as share the guests memory over a memfd.
+
+::
+
+  host# qemu-system								\
+      -chardev socket,path=$(PATH)/rng.sock,id=rng0				\
+      -device vhost-user-rng-pci,chardev=rng0					\
+      -m 4096 									\
+      -object memory-backend-file,id=mem,size=4G,mem-path=/dev/shm,share=on	\
+      -numa node,memdev=mem							\
+      ...