Fix Build warnings

[andreeaflorescu]

cargo build

warning: #[derive] can't be used on a non-Copy #[repr(packed)] struct (error E0133)
--> virtio_sys/src/virtio_net.rs:602:10
|
602 | #[derive(Debug, Default)]

warning: unused import: std::ascii::AsciiExt
--> devices/src/virtio/block.rs:531:9
|
531 | use std::ascii::AsciiExt;
| ^^^^^^^^^^^^^^^^^^^^
|
= note: #[warn(unused_imports)] on by default

[raduweiss]

Apparently, these will become errors in a future Rust release, so +1 for fixing them now.

[dianpopa]

Fixed by #245 . Closing...