intel_iommu: support passthrough (PT)

intel_iommu: support passthrough (PT)

Hardware support for VT-d device passthrough. Although current Linux can
live with iommu=pt even without this, but this is faster than when using
software passthrough.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Liu, Yi L <yi.l.liu@linux.intel.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>