Having experimented with many edge cases of running multiple build.sh
commands in parallel / against busy virtualbox setups, the only really
reliable way to produce consistent images is to not do these commands
in parallel and to not do them while the machine is doing many other
things.
If virtualbox or the machine that hosts it is very busy, and/or it has
a lot of disks it knows/knew about, and/or its tuesday, behavior may
be a bit different.
Realizing this reality, this commit adds some scripts that try really
hard to set virtualbox back to known/healthy state before building.