Check non-default feature code and tests in vhost-ci/clippy-x86

[keiichiw]

The current clippy-x86 builder in vhost-ci doesn't cover code for non-default features (e.g. vhost-kern, vhost-user, etc). So, we have some clippy errors there now, which will be fixed by #23.
To improve code quality, it would be nice if we can pass more flags in clippy there like clippy --all-targets --all-features.

[andreeaflorescu]

These should be added to a custom pipeline, similar to the other rust-vmm crates that have optional features.
Here is an example of a custom pipeline: https://github.com/rust-vmm/event-manager/blob/master/.buildkite/custom-pipeline.yml

The cargo clippy with more flags is already tracked to be added to all crates here: rust-vmm/rust-vmm-ci#21

[keiichiw]

@jiangliu I think we need to update Buildkite as @andreeaflorescu commented at #26 (comment).
Can I ask you to do it?