doc: develop: Convert README.virtio to reST

This converts the existing README.virtio to reST, and puts it under
the develop/driver-model/ directory.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/develop/driver-model/index.rst b/doc/develop/driver-model/index.rst
index fd4575d..10a7625 100644
--- a/doc/develop/driver-model/index.rst
+++ b/doc/develop/driver-model/index.rst
@@ -27,3 +27,4 @@
    soc-framework
    spi-howto
    usb-info
+   virtio