Refactor interface selection

[JayFoxRox]

I don't remember how the interface function overloads / replacing original functions work, but it's quite hacky.

There should be an object oriented interface to each backend, and we should create an object to access the xbox with that specific interface.
I have a fake interface in nv2a-trace already

There should also be a helper interface to auto-decide the interface for the user (this is the primary job of xboxpy.py: convience loader for gdb sessions, as xboxpy is not in module path).