VT-d: remove Xen and tboot range from dom0's VT-d table

VT-d: remove Xen and tboot range from dom0's VT-d table

This a step forward to fix the security hole introduced by dom0's 1:1
mapping VT-d table: remove the critical code and data from it. The
more flexible solution is to update dom0's VT-d table on demand as what
will be done for other PV domains. However, there could bring a
performance issue even with software optimization. Iotlb flush of some
hardware is time-consuming.

Signed-off-by: Yang, Xiaowei <xiaowei.yang@intel.com>