net: support multiple interfaces

[matttbe]

virtme-run supports multiple --net arguments to setup virtio-net devices, but the init program was only starting the DHCP client for one of them: the first one it discovered (which here in rust was not necessarily the first one by alphabetical order).

Now, the DHCP client is started for each interface, so each of them can have an assigned IP address.

Technically, instead of returning one network device, a vector of devices is returned. Same for the threads to start the DHCP clients.

While at it, the loopback interface is also setup in a thread: it is fine if nothing else is depending on it. (Or maybe Snap depends on it? I didn't check with my current setup not having Snap. But if it does, I guess the modifications in the main() function are not correct: lo should be setup, then the vector should be created.)

Please note that I still need to learn Rust, and it is the first time I'm writing "useful" code. Do not hesitate to tell me if there are stuff to improve :)

[matttbe]

While at it, I also just fixed a warning that was present before my modifications:

  warning: irrefutable `if let` pattern
     --> src/main.rs:206:8
      |
  206 |     if let Err(err) = reboot::reboot(reboot::RebootMode::RB_POWER_OFF) {
      |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      |
      = note: this pattern will always match, so the `if let` is useless
      = help: consider replacing the `if let` with a `let`
      = note: `#[warn(irrefutable_let_patterns)]` on by default
      
  warning: `virtme-ng-init` (bin "virtme-ng-init") generated 1 warning

[arighi]

Looks good to me, thanks!