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

[ludfjig]

#1093 introduced

struct GuestPageTableBuffer {
    buffer: std::cell::RefCell<Vec<u8>>,
}.

To avoid UB in the into_bytes implementation, this buffer made Vec<u8> instead of Vec<u64>. 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<u64>