Remove possibility of silent errors in `vmem::TableOps`

#1093 introduced
```rust
struct GuestPageTableBuffer {
buffer: std::cell::RefCell>,
}.
```
To avoid UB in the `into_bytes` implementation, this buffer made `Vec` instead of `Vec`. However this forced `read_entry` to possibly error silently using `unwrap_or(0)`.

We should remove the possibility of error occurring silently, either by panicking, or by rewriting `into_bytes` to avoid UB using `Vec`