# File lib/vagrant-libvirt/config.rb, line 294
      def _get_cdrom_dev(cdroms)
        exist = Hash[cdroms.collect { |x| [x[:dev], true] }]
        # hda - hdc
        curr = 'a'.ord
        while curr <= 'd'.ord
          dev = 'hd' + curr.chr
          if exist[dev]
            curr += 1
            next
          else
            return dev
          end
        end

        # is it better to raise our own error, or let libvirt cause the exception?
        raise 'Only four cdroms may be attached at a time'
      end