run_qemu: make git-qemu argument a path

[nmtadam]

Remove hardcoded git-qemu path, if the argument is not provided and needed for cxl options then set ~/git/qemu/ as the default

Signed-off-by: Adam Manzanares [email protected]

[stellarhopper]

I forgot to add above - if --git-qemu=~/git/qemu becomes the new default, we'd need a new option to use system installed qemu. Maybe we just add a new boolean option for that, --system-qemu