XBDM interface incompatible with older versions of XBDM

Installing the XBDM hack requires `GetMem2` (called from `read_u32` in `resolve_export`).

[This was only added in version 5028.](http://xboxdevwiki.net/XBDM_commands_by_version)

A fallback in xboxpy to GetMem exists, but it's disabled.
We should also warn the user because this has severe performance implications.