pci: implement bridge filtering

Support bridge filtering on top of the memory
API as suggested by Avi Kivity:

Create a memory region for the bridge's address space.  This region is
not directly added to system_memory or its descendants.  Devices under
the bridge see this region as its pci_address_space().  The region is
as large as the entire address space - it does not take into account
any windows.

For each of the three windows (pref, non-pref, vga), create an alias
with the appropriate start and size.  Map the alias into the bridge's
parent's pci_address_space(), as subregions.

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

hw/pci_internals.h

@@ -41,6 +41,9 @@ struct PCIBridge {
 
     /* private member */
     PCIBus sec_bus;
+    MemoryRegion alias_pref_mem;
+    MemoryRegion alias_mem;
+    MemoryRegion alias_io;
     pci_map_irq_fn map_irq;
     const char *bus_name;
 };