Avoid function pointer casts

[JayFoxRox]

Fixes warnings about casts; this has come up in #22, but was then removed again

I did not test this and do not plan to fix it if it's broken. If someone else wants to finish and PR this code, please do so.

Having worked with this codebase, the code-quality is quite horrible. It's probably easier to make our own tool in the future.

[GXTX]

I have not tested further than creating a iso and running it in XQEMU, this doesn't seem to break anything.