VT-d: prevent dom0 to use VT-d HW

VT-d: prevent dom0 to use VT-d HW

pv-ops dom0 contains Linux upstream VT-d driver, and will go to enable
it when VT-d is set in kernel config file. It should not enable VT-d
in dom0.

Currently it already zaps ACPI DMAR signature to prevents dom0 using
VT-d HW when VT-d is enabled for Xen. But when VT-d is not enabled for
Xen, and VT-d is set in pv-ops kernel config file, pv-ops dom0 will go
to enable it. This will results in pv-ops dom0 booting failure. This
patch prevents dom0 to use VT-d HW whether VT-d is enabled or disabled
for Xen.

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