intel_iommu: Drop extended root field

intel_iommu: Drop extended root field

VTD_RTADDR_RTT is dropped even by the VT-d spec, so QEMU should
probably do the same thing (after all we never really implemented it).
Since we've had a field for that in the migration stream, to keep
compatibility we need to fill the hole up.

Please refer to VT-d spec 10.4.6.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20190329061422.7926-3-peterx@redhat.com>
Reviewed-by: Liu, Yi L <yi.l.liu@intel.com>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>