docs/devel: start documenting writing VirtIO devices

docs/devel: start documenting writing VirtIO devices

While writing my own VirtIO devices I've gotten confused with how
things are structured and what sort of shared infrastructure there is.
If we can document how everything is supposed to work we can then
maybe start cleaning up inconsistencies in the code.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20220309164929.19395-1-alex.bennee@linaro.org>

Message-Id: <20220321153037.3622127-10-alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>