qemu: Make Ubuntu images buildable again with autopkgtest-build-qemu
Don't attempt to install ifupdown and rng-tools, which are only available in universe.
autopkgtest-buildvm-ubuntu-cloud is the preferred tool but some workflows might still rely on autopkgtest-build-qemu.
This supersedes !309 (closed) and the core of !310 (closed) with the sole goal of being a quick fix to get images to build again. It turns out that some workflows do still rely on this, for example the qemu
backend of debci
uses this.