KVM: x86/mmu: Explicitly disallow private accesses to emulated MMIO

Explicitly detect and disallow private accesses to emulated MMIO in
kvm_handle_noslot_fault() instead of relying on kvm_faultin_pfn_private()
to perform the check.  This will allow the page fault path to go straight
to kvm_handle_noslot_fault() without bouncing through __kvm_faultin_pfn().

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240228024147.41573-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>