QEMU on aarch64 requires an explicit machine type (e.g., '-machine virt'),
unlike x86_64 which has a default. Previously, the test script hardcoded
'qemu-kvm' without machine type arguments, causing aarch64 tests to fail
with "No machine specified, and there is no default".
By importing and using nixpkgs' qemu-common.nix library, we reuse the
existing platform-specific QEMU configuration logic.
Originally this was manually applied with;
```sh
nixpkgs-fmt **.nix && statix fix
```
but I overlooked the fact that `**.nix` would only expand to files in
the root (I should have used `**/*.nix`).
Previous commit adds `nix fmt` support which passes `.` to `nixpkgs-fmt`
(if no other path is explicitly specified when running `nix fmt`).
This commit includes the changes made by running `nix fmt`.
As discussed in [this
comment](https://github.com/nix-community/disko/pull/143#discussion_r1097912402),
as a blanket rule converting everything possible to `inherit` like
statements can hurt readability.
Add config file for statix to disable these checks and fixes, then rerun
the autofix with these options.
Apply standard formatting and some statix conventions using;
```sh
nixpkgs-fmt **.nix && statix fix .
```
With the intent of making contribution a bit easier and reducing mental
load in hand formatting (in the same vein as [black]).
[black]: https://github.com/psf/black#the-uncompromising-code-formatter
