KVM: x86/mmu: Consolidate "is MMIO SPTE" code

Replace the open-coded "is MMIO SPTE" checks in the MMU warnings
related to software-based access/dirty tracking to make the code
slightly more self-documenting.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>