Usually Vagrant uses Virtualbox by default but how about if I want to run Libvirt instead?

# -*- mode: ruby -*-
# vi: set ft=ruby :

Vagrant.configure("2") do |config|
  config.vm.box = "debian/testing64"
  config.vm.provision "shell", path: "vagrant.sh"
end

Requirements.

sudo apt install vagrant libvirt

Running.

vagrant up --provider=libvirt