Add demo for xvfb.

[rpdedeus]

Add demo for xvfb.

Your checklist for this pull request

🚨 Please review the guidelines for contributing to this repository.

• Make sure you are requesting to pull a topic/feature/bugfix branch (right side) and not your master branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or in Jenkins JIRA
• Link to relevant pull requests, esp. upstream and downstream changes
• Did you provide a test-case? That demonstrates feature works or fixes the issue.

[codecov]

Codecov Report

Merging #2202 (19b8364) into master (aa06e58) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2202   +/-   ##
=========================================
  Coverage     80.96%   80.96%           
  Complexity      843      843           
=========================================
  Files            72       72           
  Lines          2506     2506           
  Branches        352      352           
=========================================
  Hits           2029     2029           
  Misses          365      365           
  Partials        112      112           

[timja]

Please follow the contributing process for new demoes https://github.com/jenkinsci/configuration-as-code-plugin/blob/master/demos/README.adoc#contributing