pmb.qemu: cleanup, refactoring, use consistent hardware, remove vexpress

This is a larger set of various cleanup/refactoring of the QEMU code in pmbootstrap to make QEMU great again.

I have split it into neatly separated commits with explanations (which might be a bit too long), so it's probably better to review the commits rather than the final changes.

This depends on related changes in pmaports: pmaports!1029 (merged)

Closes #1836 (closed)