We need a single definition of the hypervisor build configuration options, so create a script to generate the correct set to avoid having multiple (differing) sets of optionss in various scripts/config files.

This is a replacement for clearcontainers/tests#339: it makes more sense to host this script in the packaging repo and have other repos (such as https://github.com/clearcontainers/tests), download just this script for building qemu* under CI for example.