Also create piix3 property when Xen is enabled

This recently added line in hw/pc_piix.c is causing a SEGV on a Xen
setup because the piix3 property is never created:

qdev_property_add_child(qdev_resolve_path("/i440fx/piix3", NULL),
                            "rtc", (DeviceState *)rtc_state, NULL);
Signed-off-by: NJulian Pidancet <julian.pidancet@gmail.com>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

hw/piix_pci.c

@@ -324,9 +324,8 @@ static PCIBus *i440fx_common_init(const char *device_name,
                 pci_create_simple_multifunction(b, -1, true, "PIIX3"));
         pci_bus_irqs(b, piix3_set_irq, pci_slot_get_pirq, piix3,
                 PIIX_NUM_PIRQS);
-
-        qdev_property_add_child(dev, "piix3", &piix3->dev.qdev, NULL);
     }
+    qdev_property_add_child(dev, "piix3", &piix3->dev.qdev, NULL);
     piix3->pic = pic;
 
     (*pi440fx_state)->piix3 = piix3;