virtualizor.py: doc prepare_build_dir.sh script
virtualizor.py: doc prepare_build_dir.sh script

Add a little inline documentation on top of `prepare_build_dir.sh`
script.

redhat-cip/infra-virt,redhat-cip/infra-virt,goneri/infra-virt,enovance/infra-virt,enovance/infra-virt,goneri/infra-virt