xen: add the Xenbus sysfs and virtual device hotplug driver

This communicates with the machine control software via a registry
residing in a controlling virtual machine. This allows dynamic
creation, destruction and modification of virtual device
configurations (network devices, block devices and CPUS, to name some
examples).

[ Greg, would you mind giving this a review?  Thanks -J ]

Signed-off-by: Ian Pratt <ian.pratt@xensource.com>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Cc: Greg KH <greg@kroah.com>
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index eb42b52..56592f0 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -1 +1,2 @@
 obj-y	+= grant-table.o
+obj-y	+= xenbus/