vagrant: Add vagrant-libvirt to the gemset

With this, you can "install" the plugin just by adding the following to
~/vagrant.d/plugins.json:

  {
    "version":"1",
    "installed": {
      "vagrant-libvirt": {
        "ruby_version":"2.5.3",
        "vagrant_version":"2.2.0",
        "gem_version":"",
        "require":"",
        "sources":[]
      }
    }
  }