PCI: Investigate `clippy::large_enum_variant`

```
warning: large size difference between variants
--> src/arch/x86_64/kernel/pci.rs:120:1
|
120 | / pub enum PciDriver<'a> {
121 | | VirtioFs(SpinlockIrqSave>),
| | --------------------------------------------- the largest variant contains at least 384 bytes
122 | | VirtioNet(SpinlockIrqSave),
| | ------------------------------------------- the second-largest variant contains at least 0 bytes
123 | | RTL8139Net(SpinlockIrqSave),
124 | | }
| |_^ the entire enum is at least 0 bytes
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#large_enum_variant
= note: `#[warn(clippy::large_enum_variant)]` on by default
help: consider boxing the large fields to reduce the total size of the enum
|
121 | VirtioFs(Box>>),
|
```