hvm/monitor: fix usage of the control register mask

Previous usage is not correct and would prevent certain updates from
being notified to the monitor client.

For example if (value ^ old) == (PGE | PSE) and mask == PGE this
update would not be notified.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>

xen/arch/x86/hvm/monitor.c

@@ -39,7 +39,7 @@ bool hvm_monitor_cr(unsigned int index, unsigned long value, unsigned long old)
     if ( (ad->monitor.write_ctrlreg_enabled & ctrlreg_bitmask) &&
          (!(ad->monitor.write_ctrlreg_onchangeonly & ctrlreg_bitmask) ||
           value != old) &&
-         (!((value ^ old) & ad->monitor.write_ctrlreg_mask[index])) )
+         ((value ^ old) & ~ad->monitor.write_ctrlreg_mask[index]) )
     {
         bool sync = ad->monitor.write_ctrlreg_sync & ctrlreg_bitmask;