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/Kconfig b/drivers/Kconfig
index d945ffc..f4076d9 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -93,4 +93,6 @@
 source "drivers/kvm/Kconfig"
 
 source "drivers/uio/Kconfig"
+
+source "drivers/virtio/Kconfig"
 endmenu