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/net/xen-netfront.c b/drivers/net/xen-netfront.c
index baa051d..a869b45 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -42,6 +42,7 @@
 #include <linux/mm.h>
 #include <net/ip.h>
 
+#include <xen/xen.h>
 #include <xen/xenbus.h>
 #include <xen/events.h>
 #include <xen/page.h>