riscv/virt: add virt_nsh with docs

This adds `rv-virt:virt_nsh` config w/ docs for using virtio serial
as NuttX console.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

Documentation/platforms/risc-v/qemu-rv/boards/rv-virt/index.rst

@@ -359,6 +359,26 @@ flats64
 Similar to the `nsh`_ configuration, but running in S-mode.
 This configuration is used for 64-bit RISC-V
 
+virt_nsh
+--------
+
+Similar to `nsh`_ configuration, but uses virtio serial device as console.
+Use it below with QEMU::
+
+    $ qemu-system-riscv32 -M virt,aclint=on -nographic \
+    -chardev socket,id=aux,path=/tmp/aux,server=on,wait=on \
+    -device virtio-serial-device,bus=virtio-mmio-bus.0 \
+    -device virtconsole,chardev=aux \
+    -bios nuttx
+
+Then from another terminal, use below command to access the console::
+
+    $ socat UNIX-CLIENT:/tmp/aux -
+
+We can finish the session with ``quit`` command in NSH session.
+
+Note the above command line uses UNIX domain socket so please change the socket parameters on hosts without UNIX domain socket.
+
 RISC-V GDB Debugging
 ====================