KVM: x86/mmu: Use Accessed bit even when _hardware_ A/D bits are disabled

KVM: x86/mmu: Use Accessed bit even when _hardware_ A/D bits are disabled

Use the Accessed bit in SPTEs even when A/D bits are disabled in hardware,
i.e. propagate accessed information to SPTE.Accessed even when KVM is
doing manual tracking by making SPTEs not-present.  In addition to
eliminating a small amount of code in is_accessed_spte(), this also paves
the way for preserving Accessed information when a SPTE is zapped in
response to a mmu_notifier PROTECTION event, e.g. if a SPTE is zapped
because NUMA balancing kicks in.

Note, EPT is the only flavor of paging in which A/D bits are conditionally
enabled, and the Accessed (and Dirty) bit is software-available when A/D
bits are disabled.

Note #2, there are currently no concrete plans to preserve Accessed
information.  Explorations on that front were the initial catalyst, but
the cleanup is the motivation for the actual commit.

Link: https://lore.kernel.org/r/20241011021051.1557902-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>