pv-on-hvm: Use wmb instead of rmb

diff -r f681c4de91fc unmodified_drivers/linux-2.6/platform-pci/evtchn.c
@@ -284,7 +284,7 @@
 
 #ifndef CONFIG_X86 /* No need for a barrier -- XCHG is a barrier on x86. */
 	/* Clear master flag /before/ clearing selector flag. */
-	rmb();
+	wmb();
 #endif
 	l1 = xchg(&v->evtchn_pending_sel, 0);