Make piuparts more flexible for the user, easier to extend, and have it support KVM

This spec proposes to improve it by writing automated test suites for better robustness, implementing the use of KVM in addition to chroot for virtualization, and adding a plugin system to allow more flexible testing. The new version of piuparts will use ideas and code from Michael Vogt's update-manager, Ian Haken's vlosuts, and Ian Jackson's autopkgtest, or add code to integrate those tools.