virtio: Add block driver support

This adds virtio block device driver support.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
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 e20dd69..b72477a 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -36,4 +36,11 @@
 	  This is the virtual net driver for virtio. It can be used with
 	  QEMU based targets.
 
+config VIRTIO_BLK
+	bool "virtio block driver"
+	depends on VIRTIO
+	help
+	  This is the virtual block driver for virtio. It can be used with
+	  QEMU based targets.
+
 endmenu