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}"