xen: Fix xen_enabled().

Use the "host" CONFIG_ define instead of the "target" one.
Signed-off-by: NAnthony PERARD <anthony.perard@citrix.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>

hw/xen.h

@@ -24,7 +24,7 @@ extern int xen_allowed;
 
 static inline int xen_enabled(void)
 {
-#ifdef CONFIG_XEN
+#ifdef CONFIG_XEN_BACKEND
     return xen_allowed;
 #else
     return 0;