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/char/hvc_xen.c b/drivers/char/hvc_xen.c
index eba999f..93d33816 100644
--- a/drivers/char/hvc_xen.c
+++ b/drivers/char/hvc_xen.c
@@ -25,6 +25,8 @@
 #include <linux/types.h>
 
 #include <asm/xen/hypervisor.h>
+
+#include <xen/xen.h>
 #include <xen/page.h>
 #include <xen/events.h>
 #include <xen/interface/io/console.h>