Implement QEMU-based porter boxes

All currently active CI workers run their tests in non-interactive QEMU VMs (via autopkgtest).

These hosts could just as easily provide interactive VMs into which other users could connect to via SSH, just like any other porterbox.

This would be even more efficient and less cumbersome than regular porterboxes, as there is no need for setting up chroots (we have pre-built images for various distributions).

The difficulty is arbitration: a GPU that is in active use by a running autopkgtest cannot be used as a porterbox, and vice versa.