Ensure box version default of 0 uses mtime (#1512)

If a box is added directly without a version, vagrant will default set
it to 0. When this occurs it is necessary to ensure use of mtime to
distinguish when the disk image in the box was last updated to allow
replacement of the box to trigger a fresh upload.

Fixes: #1382

lib/vagrant-libvirt/action/handle_box_image.rb

@@ -134,7 +134,7 @@ module VagrantPlugins
                       ''
                     end
 
-          if version.empty?
+          if version.empty? || version == '0'
             ui.warn(I18n.t('vagrant_libvirt.box_version_missing', name: box.name.to_s))
 
             version = "0_#{File.mtime(path).to_i}"

spec/unit/action/handle_box_image_spec.rb

@@ -125,6 +125,23 @@ describe VagrantPlugins::ProviderLibvirt::Action::HandleBoxImage do
         end
       end
 
+      context 'when box version set to 0' do
+        let(:box_mtime) { Time.now }
+
+        before do
+          expect(env[:machine]).to receive_message_chain("box.version") { '0' }
+          expect(File).to receive(:mtime).and_return(box_mtime)
+        end
+
+        it 'should use the box file timestamp' do
+          expect(ui).to receive(:warn).with(/No verison detected for test/)
+
+          expect(subject.call(env)).to be_nil
+          expect(env[:box_volume_number]).to eq(1)
+          expect(env[:box_volumes]).to match([hash_including({:name=>"test_vagrant_box_image_0_#{box_mtime.to_i}_box.img"})])
+        end
+      end
+
       context 'When config.machine_virtual_size is set and smaller than box_virtual_size' do
         before do
           env[:machine].provider_config.machine_virtual_size = 1