Enable frozen string across project (#1319)

Turn on frozen string support in all files by using a comment to avoid
enabling across dependencies where it may not work.

Fixes: #1177

vagrant-libvirt.gemspec

@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: true
 require File.expand_path('../lib/vagrant-libvirt/version', __FILE__)
 
 Gem::Specification.new do |s|