KVM: x86/mmu: Add shadow mask for effective host MTRR memtype

Add shadow_memtype_mask to capture that EPT needs a non-zero memtype mask
instead of relying on TDP being enabled, as NPT doesn't need a non-zero
mask.  This is a glorified nop as kvm_x86_ops.get_mt_mask() returns zero
for NPT anyways.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20220715230016.3762909-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>