pci_bridge: Use a default map_irq function

The PCI bridge spec defines a default swizzle for translating INTx
IRQs from secondary bus to primary.  Use this by default for any
bridge that doesn't set a function.
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/pci/pci_bridge.c

@@ -366,7 +366,7 @@ int pci_bridge_initfn(PCIDevice *dev)
     qbus_create_inplace(&sec_bus->qbus, TYPE_PCI_BUS, &dev->qdev,
                         br->bus_name);
     sec_bus->parent_dev = dev;
-    sec_bus->map_irq = br->map_irq;
+    sec_bus->map_irq = br->map_irq ? br->map_irq : pci_swizzle_map_irq_fn;
     sec_bus->address_space_mem = &br->address_space_mem;
     memory_region_init(&br->address_space_mem, "pci_bridge_pci", INT64_MAX);
     sec_bus->address_space_io = &br->address_space_io;