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/balloon.c b/drivers/xen/balloon.c
index d31505b..826dda4 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -52,6 +52,8 @@
 
 #include <asm/xen/hypervisor.h>
 #include <asm/xen/hypercall.h>
+
+#include <xen/xen.h>
 #include <xen/interface/xen.h>
 #include <xen/interface/memory.h>
 #include <xen/xenbus.h>