VT-d: fix iommu_domid for PCI/PCIx devices assignment

VT-d: fix iommu_domid for PCI/PCIx devices assignment

Currently, it clears iommu_domid and domid_map at the end of
domain_context_unmap_one() if no other devices under the same iommu
owned by this domain. But, when assign a PCI/PCIx device to a guest,
it also assigns its upstream bridge to the guest, and they use the
same iommu_domid. In the deassignment, the iommu_domid and domid_map
are cleared in domain_context_unmap_one() for the assigned PCI/PCIx
device, therefore it cannot get valid iommu_domid in followed
domain_context_unmap_one for its upstream bridge. It causes PCI/PCIx
device re-assignment failure.

This patch moves the iommu_domid and domid_map clearing code to the
end of domain_context_unmap, where all dependent
domain_context_unmap_one()s are completed, thus fix above issue.

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