hw/pxb: introduce pxb-pcie expander for PCIe machines

The pxb-pcie is the counterpart of pxb for PCI express machines.
The new device re-uses the pxb code, but appears to the guests
as a different device. The pxb-pcie device does not have an internal
pci-pci bridge and exposes a PCIe root bus instead of a PCI one.

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>
