hw/pci: introduce PCI Expander Bridge (PXB)

PXB is a "light-weight" host bridge whose purpose is to enable
the main host bridge to support multiple PCI root buses
for pc machines.

As oposed to PCI-2-PCI bridge's secondary bus, PXB's bus
is a primary bus and can be associated with a NUMA node
(different from the main host bridge) allowing the guest OS
to recognize the proximity of a pass-through device to
other resources as RAM and CPUs.

The PXB is composed from:
 - A primary PCI bus (can be associated with a NUMA node)
   Acts like a normal pci bus and from the functionality point
   of view is an "expansion" of the bus behind the
   main host bridge.
 - A pci-2-pci bridge behind the primary PCI bus where the actual
   devices will be attached.
 - A host-bridge PCI device
   Situated on the bus behind the main host bridge, allows
   the BIOS to configure the bus number and IO/mem resources.
   It does not have its own config/data register for configuration
   cycles, this being handled by the main host bridge.
-  A host-bridge sysbus to comply with QEMU current design.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>