https://github.com/rust-vmm/vm-memory/pull/260 introduced error cases to resolve https://github.com/rust-vmm/vm-memory/issues/269 but did not cover these with regression tests. Writes tests that exercise the paths where `GuestMemory::try_access` returns `Err(Error::CallbackOutOfRange)` and `Err(Error::GuestAddressOverflow)`.
