KVM: nVMX: Add a sanity check that nested PML Full stems from EPT Violations

Add a WARN_ON_ONCE() sanity check to verify that a nested PML Full VM-Exit
is only synthesized when the original VM-Exit from L2 was an EPT Violation.
While KVM can fallthrough to kvm_mmu_do_page_fault() if an EPT Misconfig
occurs on a stale MMIO SPTE, KVM should not treat the access as a write
(there isn't enough information to know *what* the access was), i.e. KVM
should never try to insert a PML entry in that case.

Link: https://lore.kernel.org/r/20240209221700.393189-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>