[Libguestfs] CI for libnbd is now active

I have (finally) pushed the CI enablement patches upstream and can see the pipeline running already here: https://gitlab.com/nbdkit/libnbd/-/pipelines Clicking on the status opens it up, amongst other clickable objects are the stage icons. I managed to fix the easier issues, but there are still more which I do not know how to solve (and for some of them I tried more than I should've). There's a small write-up, partially copied from libvirt, in ci/README.rst, feel free to change the format. The patch on top of libvirt-ci that was used for generating the initial CI assets in ci/ was submitted as a merge request here: https://gitlab.com/libvirt/libvirt-ci/-/merge_requests/120 and that is were dependency changes need to be tracked. I'm glad to answer any questions. Have a nice day, Martin