KVM: x86/mmu: Don't install bogus MMIO SPTEs if MMIO caching is disabled

If MMIO caching is disabled, e.g. when using shadow paging on CPUs with
52 bits of PA space, go straight to MMIO emulation and don't install an
MMIO SPTE.  The SPTE will just generate a !PRESENT #PF, i.e. can't
actually accelerate future MMIO.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>