VT-d: pci code cleanup

VT-d: pci code cleanup

This patch moves the pci code from iommu.c to pci.c. Instead of setup
pci hierarchy in array bus2bridge in iommu_context_mapping, use
scan_pci_devices once to add all existed PCI devices in system to
alldevs_list and setup pci hierarchy in array bus2bridge. In addition,
implement find_upstream_bridge to find the upstream PCIe-to-PCI/PCIX
bridge or PCI legacy bridge for a PCI device, therefore it's cleanly
to handle context map/unmap for PCI device, even for source-id
setting.

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