xen: separate out frontend xenbus

Impact: refactor

Make a distinct frontend xenbus, in preparation for adding a backend xenbus.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
[corresponds to 2fd433a4188f in git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git
 with adjustments to reflect changes in the code which is moved]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/Kconfig

diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
index 4b9359a..83c32cb 100644
--- a/drivers/block/Kconfig
+++ b/drivers/block/Kconfig
@@ -464,6 +464,7 @@
 	tristate "Xen virtual block device support"
 	depends on XEN
 	default y
+	select XEN_XENBUS_FRONTEND
 	help
 	  This driver implements the front-end of the Xen virtual
 	  block device driver.  It communicates with a back-end driver