virtio: Allow virtio to be modular and used by modules

This is needed for the virtio PCI device to be compiled as a module.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
index 9e33fc4..de0c8c2 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -1,8 +1,8 @@
 # Virtio always gets selected by whoever wants it.
 config VIRTIO
-	bool
+	tristate
 
 # Similarly the virtio ring implementation.
 config VIRTIO_RING
-	bool
+	tristate
 	depends on VIRTIO