xen: move Xen-testing predicates to common header

Move xen_domain and related tests out of asm-x86 to xen/xen.h so they
can be included whenever they are necessary.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
diff --git a/drivers/xen/xenfs/super.c b/drivers/xen/xenfs/super.c
index 6559e0c..8924d93 100644
--- a/drivers/xen/xenfs/super.c
+++ b/drivers/xen/xenfs/super.c
@@ -13,6 +13,8 @@
 #include <linux/fs.h>
 #include <linux/magic.h>
 
+#include <xen/xen.h>
+
 #include "xenfs.h"
 
 #include <asm/xen/hypervisor.h>