`Uninitialized::new` should take guest binary by ref

So one GuestBinary can be used for many different sandboxes

#559 made some progress on this, which at leasts avoid copying the inner buffer.
#560 was closed due to #605 changing the API, making it harder to use the `Borrow` trait as initially intended.

Start by reading issues #559, #560, and #605, then locate `Uninitialized::new` and its `GuestBinary` call sites. The change is complete when one `GuestBinary` can be passed by reference and reused to create multiple sandboxes without copying its inner buffer.

Written by the indexing model from the issue text.