Fix warnings with rust 1.37

[petreeftime]

Reason for This PR

Rust 1.37 warns about using Trait instead of dyn Trait (rust-lang/rust#61203) and inclusive ranges with ... syntax ranges instead of ..= syntax (rust-lang/rust#28237).

Description of Changes

I've fixed the specific instances where these two warnings appear in the firecracker code.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license.

PR Checklist

[Author TODO: Meet these criteria. Where there are two options, keep one.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• Either this PR is linked to an issue, or, the reason for this PR is
  clearly provided.
• The description of changes is clear and encompassing.
• Either no docs need to be updated as part of this PR, or, the required
  doc changes are included in this PR. Docs in scope are all *.md files
  located either in the repository root, or in the docs/ directory.
• Either no code has been touched, or, code-level documentation for touched
  code is included in this PR.
• Either no API changes are included in this PR, or, the API changes are
  reflected in firecracker/swagger.yaml.
• Either the changes in this PR have no user impact, or, the changes in
  this PR have user impact and have been added to the CHANGELOG.md file.