KVM: x86/mmu: Add a dedicated flag to track if A/D bits are globally enabled

KVM: x86/mmu: Add a dedicated flag to track if A/D bits are globally enabled

Add a dedicated flag to track if KVM has enabled A/D bits at the module
level, instead of inferring the state based on whether or not the MMU's
shadow_accessed_mask is non-zero.  This will allow defining and using
shadow_accessed_mask even when A/D bits aren't used by hardware.

Link: https://lore.kernel.org/r/20241011021051.1557902-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>