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/templates/domain_template.xml.j2

@@ -25,7 +25,7 @@
   </clock>
   <devices>
     <disk type='file' device='disk'>
-      <driver name='qemu' type='qcow2'/>
+      <driver name='qemu' type='qcow2' discard='unmap'/>
       <source file='/var/lib/libvirt/images/$vm_name.qcow2'/>
       <target dev='sdb' bus='scsi'/>
     </disk>