virtio: Add a Sandbox transport driver

This driver provides support for Sandbox implementation of virtio
transport driver which is used for testing purpose only.

Two drivers are provided. The 2nd one is a driver that lacks the
'notify' op.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
3 files changed