libvirt-executor improvements

A bunch of improvements to the libvirt-executor, with the major one being that the image is now built in the arch-boxes projects (arch-boxes!200 (merged)), instead of on each runner.

Please see the commit messages for more details.

---

Deployed on runner1.archlinux.org and secure-runner1.archlinux.org.

roles/gitlab_runner/tasks/main.yml

@@ -79,7 +79,7 @@
       copy: src={{ item.src }} dest={{ item.dest }} owner=root group=root mode={{ item.mode }}
       loop:
         - {src: arch-boxes.asc, dest: /usr/local/lib/libvirt-executor/, mode: 644}
-        - {src: domain_template.xml, dest: /usr/local/lib/libvirt-executor/, mode: 755}
+        - {src: domain_template.xml, dest: /usr/local/lib/libvirt-executor/, mode: 644}
         - {src: libvirt-executor, dest: /usr/local/bin/, mode: 755}
         - {src: libvirt-executor-update-base-image, dest: /usr/local/bin/, mode: 755}