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/evtchn.c b/drivers/xen/evtchn.c
index 79bedba..f70a4f4 100644
--- a/drivers/xen/evtchn.c
+++ b/drivers/xen/evtchn.c
@@ -48,6 +48,8 @@
 #include <linux/gfp.h>
 #include <linux/mutex.h>
 #include <linux/cpu.h>
+
+#include <xen/xen.h>
 #include <xen/events.h>
 #include <xen/evtchn.h>
 #include <asm/xen/hypervisor.h>