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/grant-table.c b/drivers/xen/grant-table.c
index 7d8f531..4c6c0bd 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -37,6 +37,7 @@
 #include <linux/vmalloc.h>
 #include <linux/uaccess.h>
 
+#include <xen/xen.h>
 #include <xen/interface/xen.h>
 #include <xen/page.h>
 #include <xen/grant_table.h>