Add argument validation checks for plugin commands

[Voxel9]

Currently, the Xbox plugin processes user-provided arguments via sscanf, however none of the data actually gets validated beforehand.

In the near future, it would be preferable to add this in. However, most people will likely use the interface instead of the command line, and seeing as the interface will always send correct commands and validate data that feeds back into it anyway, this isn't a huge breaking issue.