Fix Wii disc partitions

This is magumagu's [dvd-low-open-partition](https://github.com/magumagu/dolphin/tree/dvd-low-open-partition) rebased on top of a recent version of master, requested by JMC47. Fixes Masterpieces in Super Smash Bros. Brawl.
I can make it check if it exceeds some size that is much bigger than that, like 4 MB.