Switch to triggering CI pipelines from merge requests instead of push

This series updates libvirt-glib.git with the CI changes proposed in libvirt-ci!320 (merged)

For the sake of illustration & review, I've split this change up so that each commit] in libvirt-glib.git matches a commit in libvirt-ci.git manifest generator code. Assuming the libvirt-ci.git changes are approved, then conversions of other projects would NOT have this split series as it is is way too much work to split it up - they would just have a big bang update to latest libvirt-ci.git HEAD.