Support testing against ruby 3.0 (#1243)

Retrieve vagrant and ruby-libvirt dependencies and modify as needed to
allow testing against ruby 3.0 until released versions support.

Use conditionals to skip steps when not needed.

Note that in order to use the locally built gem added to the cache
manually, need to disable checksums. However as all other ruby
versions will continue to use it, shouldn't be an issue as long as the
cache for ruby 3.0.0 is wiped clean before being used for anything such
as publishing.

Fixes: #1244

Gemfile

@@ -8,11 +8,12 @@ group :development do
   # gem dependency because we expect to be installed within the
   # Vagrant environment itself using `vagrant plugin`.
   vagrant_version = ENV['VAGRANT_VERSION']
-  if vagrant_version
+  if !vagrant_version.nil? && !vagrant_version.empty?
     gem 'vagrant', :git => 'https://github.com/hashicorp/vagrant.git',
-      tag: vagrant_version
+      :ref => vagrant_version
   else
-    gem 'vagrant', :git => 'https://github.com/hashicorp/vagrant.git'
+    gem 'vagrant', :git => 'https://github.com/hashicorp/vagrant.git',
+      :branch => 'main'
   end
 
   begin
@@ -30,6 +31,10 @@ group :development do
     gem 'vagrant-spec', :github => 'hashicorp/vagrant-spec', :branch => "main"
   end
 
+  if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.0.0')
+    gem 'rexml'
+  end
+
   gem 'pry'
 end