Allow uet uefs build to have a --size-gb parameter specified

[hach-que]

A user wants to build a UEFS package that contains the source code, and then build the engine inside the mounted VHD. While this isn't recommended, the issue they're running into is that the VHD doesn't end up with enough free disk space for the build to actually complete.

We should have a --size-gb parameter which can be used to explicitly override the disk size of the resulting VHD or SparseImage that is created.