KVM: x86/mmu: WARN on MMIO cache hit when emulating write-protected gfn

KVM: x86/mmu: WARN on MMIO cache hit when emulating write-protected gfn

WARN if KVM gets an MMIO cache hit on a RET_PF_WRITE_PROTECTED fault, as
KVM should return RET_PF_WRITE_PROTECTED if and only if there is a memslot,
and creating a memslot is supposed to invalidate the MMIO cache by virtue
of changing the memslot generation.

Keep the code around mainly to provide a convenient location to document
why emulated MMIO should be impossible.

Suggested-by: Yuan Yao <yuan.yao@linux.intel.com>
Link: https://lore.kernel.org/r/20240831001538.336683-23-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>