Investigate & turn back on ValidateWrite in VirtWrite

[0vercl0k]

I remember I turned this off at one point:

bool Backend_t::VirtWrite(const Gva_t Gva, const uint8_t *Buffer,
                          const uint64_t BufferSize, const bool Dirty) {
  uint64_t Size = BufferSize;
  Gva_t CurrentGva = Gva;
  while (Size > 0) {
    Gpa_t Gpa;
    // XXX: Reenable ValidateReadWrite when bug is figured out.
    const bool Translate = VirtTranslate(
        CurrentGva, Gpa, MemoryValidate_t::ValidateRead /*Write*/);

I would run into cases where I'd VirtWrite onto a PTE that wasn't set writeable; but writing over it ignoring the PTE state never lead to any weird crash so I let it be.

It'd be good to understand why that is and what's the behavior behind; it might be the case that the PF handler turns it to writeable for unknown reason so we might need to emulate that behavior as well.