Fix warning 16 (optional argument cannot be erased)
Fix warning 16 (optional argument cannot be erased)

```
File "src/hostnet/vmnet.ml", line 327, characters 30-42:
327 |   let client_negotiate ~uuid ?preferred_ip ~fd =
                                    ^^^^^^^^^^^^
Error (warning 16 [unerasable-optional-argument]): this optional argument cannot be erased.
```

Signed-off-by: David Scott <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@recoil.org>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit