pcie port: define struct PCIEPort/PCIESlot and helper functions

pcie port: define struct PCIEPort/PCIESlot and helper functions

define struct PCIEPort which represents common part
of pci express port.(root, upstream and downstream.)
add a helper function for pcie port which can be used commonly by
root/upstream/downstream port.
define struct PCIESlot which represents common part of
pcie slot.(root and downstream.) and helper functions for it.
helper functions for chassis, slot -> PCIESlot conversion.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>