```ruby
module VagrantMutate
  module Converter
    class Libvirt < Converter

      def generate_metadata
        metadata = {
          'provider' => @output_box.provider_name,
          'format'   => 'qcow2',
          'virtual_size' => ( @input_box.virtual_size.to_f / (1024 * 1024 * 1024) ).ceil
        }
      end

      def write_specific_files
        # nothing to do here
      end

    end
  end
end
```

Remove hard-coded image format and reference box instead
user
```ruby
module VagrantMutate
  module Converter
    class Libvirt < Converter

      def generate_metadata
        metadata = {
          'provider' => @output_box.provider_name,
          'format'   => @output_box.image_format,
          'virtual_size' => ( @input_box.virtual_size.to_f / (1024 * 1024 * 1024) ).ceil
        }
      end

      def write_specific_files
        # nothing to do here
      end

    end
  end
end
```
assistant