Missing type conversion for OS.Netif.add_vif?

I found that the backend.ml file generated by mirari is not exactly all right, as the compiler complains that the `id` passed to `OS.Netif.add_vif` is not of type `OS.Netif.id`.  So I guess an explicit type conversion between the original `string` and the requested `OS.Netif.id` type is needed.