virtio-pci: use size from correct structure

PIO MR registration should use size from the correct notify struct.
Doesn't affect any visible behaviour because the field values are the
same (both are 4).

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-pci.c

@@ -1520,7 +1520,7 @@ static void virtio_pci_modern_regions_init(VirtIOPCIProxy *proxy)
                           &notify_pio_ops,
                           virtio_bus_get_device(&proxy->bus),
                           "virtio-pci-notify-pio",
-                          proxy->notify.size);
+                          proxy->notify_pio.size);
 }
 
 static void virtio_pci_modern_region_map(VirtIOPCIProxy *proxy,