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>
diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
index 753912f..a9d5fd0 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -37,6 +37,14 @@
 	  This driver provides support for virtio based paravirtual device
 	  drivers over PCI.
 
+config VIRTIO_SANDBOX
+	bool "Sandbox driver for virtio devices"
+	depends on SANDBOX
+	select VIRTIO
+	help
+	  This driver provides support for Sandbox implementation of virtio
+	  transport driver which is used for testing purpose only.
+
 config VIRTIO_NET
 	bool "virtio net driver"
 	depends on VIRTIO