xen: Make XEN_BACKEND selectable by DomU

XEN_BACKEND doesn't actually depend on XEN_DOM0.  DomUs can serve
backends to other DomUs.  One example is a service VM providing network
backends.

The original Kconfig defaulted Dom0 to y and it could be disabled.  DomU
could not select the option.  With the new Kconfig, we default y for
Dom0 and n for DomU.  Either can then toggle the selection.
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/Kconfig

@@ -105,8 +105,7 @@ config XEN_DEV_EVTCHN
 
 config XEN_BACKEND
 	bool "Backend driver support"
-	depends on XEN_DOM0
-	default y
+	default XEN_DOM0
 	help
 	  Support for backend device drivers that provide I/O services
 	  to other virtual machines.