Virtio interface

This attempts to implement a "virtual I/O" layer which should allow
common drivers to be efficiently used across most virtual I/O
mechanisms.  It will no-doubt need further enhancement.

The virtio drivers add buffers to virtio queues; as the buffers are consumed
the driver "interrupt" callbacks are invoked.

There is also a generic implementation of config space which drivers can query
to get setup information from the host.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Dor Laor <dor.laor@qumranet.com>
Cc: Arnd Bergmann <arnd@arndb.de>
diff --git a/drivers/virtio/Makefile b/drivers/virtio/Makefile
new file mode 100644
index 0000000..af0d57d
--- /dev/null
+++ b/drivers/virtio/Makefile
@@ -0,0 +1 @@
+obj-$(CONFIG_VIRTIO) += virtio.o