I am using Ubuntu 14.04 and have RVM with Ruby 2.2 installed I don not like to pollute my system ruby version with development work. So I use RVM.

clone the vagrant plugin git project

$ git clone https://github.com/vagrant-libvirt/vagrant-libvirt

Build the Gem

Seems like any GEM version will do. Does not seem to need a newer ruby installed.

$ gem build vagrant-libvirt.gemspec

$ vagrant plugin install vagrant-libvirt-0.0.31.gem

Vagrant plugin status will say the plugin installed is version constrained. So this way you can tell the plugin did not come from the main rubygem repository.

$ /usr/bin/vagrant plugin list

pry (0.10.1)
pry-byebug (3.1.0)
rb-readline (0.5.3)
vagrant-libvirt (0.0.31)
  - Version Constraint: 0.0.31
vagrant-lxc (1.1.0)
vagrant-mutate (1.0.0)
vagrant-share (1.1.3, system)