Fix to mmio access + misc cleanups

[dgibson]

Hi Alex, I realize this repo hasn't been much touched in years, but I'm using some of these tests while working on SR-IOV with Kata, so I thought I might as well push these fixes back.

The main thing here is a slightly polished version of Pradipta's fix (https://github.com/bpradipt/tests/commit/d884a0c1b40ddae15d945b792da60bb0c10041b2) with an actual commit message with rationale. While I was doing that, I did some other trivial quality-of-life cleanups.