KVM: x86/mmu: Add sanity checks that KVM doesn't create EPT #VE SPTEs

Assert that KVM doesn't set a SPTE to a value that could trigger an EPT
Violation #VE on a non-MMIO SPTE, e.g. to help detect bugs even without
KVM_INTEL_PROVE_VE enabled, and to help debug actual #VE failures.

Note, this will run afoul of TDX support, which needs to reflect emulated
MMIO accesses into the guest as #VEs (which was the whole point of adding
EPT Violation #VE support in KVM).  The obvious fix for that is to exempt
MMIO SPTEs, but that's annoyingly difficult now that is_mmio_spte() relies
on a per-VM value.  However, resolving that conundrum is a future problem,
whereas getting KVM_INTEL_PROVE_VE healthy is a current problem.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240518000430.1118488-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>