`make-based`: Add option in `do.sh` scripts to get path to `qemu-system-x86_64` executable

[razvand]

Some setups may have multiple qemu-system-x86_64 executables. In that case, you want to be able to specify the exact path of the executable, and not use the default one.

Add a variable in do.sh scripts and in support scripts in include/ to have these features.

Hackathon points: 10

[pxanthopoulos]

I'm working on this

[ntua-el19097]

Pull request made

[razvand]

Hackathon-points: 10