VT-d: Fix PCI-X device assignment

VT-d: Fix PCI-X device assignment

When assign PCI device, current code just map its bridge and its
secondary bus number and devfn 0. It doesn't work for PCI-x device
assignment, because the request may be the source-id in the original
PCI-X transaction or the source-id provided by the bridge. It needs to
map the device itself, and its upstream bridges till PCIe-to-PCI/PCI-x
bridge.

In addition, add description for DEV_TYPE_PCIe_BRIDGE and
DEV_TYPE_PCI_BRIDGE for understandability.

Signed-off-by: Weidong Han <weidong.han@intel.com>