KVM: x86/mmu: Exit to userspace with -EFAULT if private fault hits emulation

Exit to userspace with -EFAULT / KVM_EXIT_MEMORY_FAULT if a private fault
triggers emulation of any kind, as KVM doesn't currently support emulating
access to guest private memory.  Practically speaking, private faults and
emulation are already mutually exclusive, but there are many flow that
can result in KVM returning RET_PF_EMULATE, and adding one last check
to harden against weird, unexpected combinations and/or KVM bugs is
inexpensive.

Suggested-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240228024147.41573-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>