Update USERS.md with Virtuozzo

[egoust]

Please ensure your pull request adheres to the following guidelines:

• For first time contributors, read Submitting a pull request
• All code is covered by unit and/or runtime tests where feasible.
• All commits contain a well written commit description including a title,
  description and a Fixes: #XXX line if the commit addresses a particular
  GitHub issue.
• If your commit description contains a Fixes: <commit-id> tag, then
  please add the commit author[s] as reviewer[s] to this issue.
• All commits are signed off. See the section Developer’s Certificate of Origin
• Provide a title or release-note blurb suitable for the release notes.
• Are you a user of Cilium? Please add yourself to the Users doc
• Thanks for contributing!

Fixes: #issue-number

<!-- Enter the release note text here if needed or remove this section! -->

[xmulligan]

/test