KVM: x86/mmu: Check for shadow-present SPTE before querying A/D status

When updating accessed and dirty bits, check that the new SPTE is present
before attempting to query its A/D bits.  Failure to confirm the SPTE is
present can theoretically cause a false negative, e.g. if a MMIO SPTE
replaces a "real" SPTE and somehow the PFNs magically match.

Realistically, this is all but guaranteed to be a benign bug.  Fix it up
primarily so that a future patch can tweak the MMU_WARN_ON checking A/D
status to fire if the SPTE is not-present.

Fixes: f8e14497 ("kvm: x86/mmu: Add access tracking for tdp_mmu")
Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>