xen/time: Return -ENODEV from xen_get_wallclock()

For any other error sync_cmos_clock() will reschedule itself
every second or so, for no good reason.
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

arch/x86/xen/time.c

@@ -74,7 +74,7 @@ static void xen_get_wallclock(struct timespec *now)
 
 static int xen_set_wallclock(const struct timespec *now)
 {
-	return -1;
+	return -ENODEV;
 }
 
 static int xen_pvclock_gtod_notify(struct notifier_block *nb,