builder/parallels: Default cdrom0 should be disconnected

This is a minor change and it doesn't break backward compatibility. 
Actually, before #1502, we had `cdrom0` device disconnected.

But now (since eb37742ab032c9cd1aa24a2669a220ebab416558) `cdrom0` is not marked as disconnected and the next error message is displayed in GUI:
![screenshot_21_10_14_16_28](https://cloud.githubusercontent.com/assets/4846154/4718464/520aedd2-5921-11e4-9fa6-ffdc7b7ca605.png)

So, this pull-request fixes that.