CI: run nightly cargo fmt wiht a custom rustfmt.toml

[stefano-garzarella]

@epilys suggested rustfmt with nightly (e.g. cargo +nightly fmt) with this rustfmt.toml to enforce several checks like the import groups.

• add rustfmt.toml in the workspace (I'm not sure if cargo fmt will use it automatically, but we can use --config-path)
• add a new step to run cargo +nightly fmt --all --check in https://github.com/rust-vmm/vhost-device/blob/main/.buildkite/custom-tests.json
• remove rustfmt.toml from vhost-device-sound and vhost-device-video

[stefano-garzarella]

@epilys will work on this.

Note: I can't assign this to you, (I don't know why). Just to let others know you're working on it, so we don't duplicate the work!

[epilys]

(I think github allows only org members to be assigned)

[andreeaflorescu]

@epilys I added you as a member of rust-vmm. You have to accept the invitation. I could also assign the issue to you. I think that's because you commented on the issue, because you are not yet a member.