Shadow_mt_mask is out of date, now it have only been used as a flag to indicate
if TDP enabled. Get rid of it and use tdp_enabled instead.

Also put memory type logical in kvm_x86_ops->get_mt_mask().
Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>