remove needless borrows

remove needless borrows

Fixes the clippy lint
```
warning: the borrowed expression implements the required traits
   --> src/medium.rs:143:9
    |
143 |         &statefile(base),
    |         ^^^^^^^^^^^^^^^^ help: change this to: `statefile(base)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrows_for_generic_args
    = note: `#[warn(clippy::needless_borrows_for_generic_args)]` on by default
```

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>