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/files/libvirt-executor

 #!/usr/bin/env bash
 set -o nounset -o errexit -o pipefail
-readonly libvirt_default_pool_path="/var/lib/libvirt/images"
+readonly libvirt_pool="images"
 
 ssh() {
   command ssh \
@@ -50,15 +50,15 @@ wait_for_ssh() {
 prepare() {
   # shellcheck disable=SC2064
   trap "exit ${SYSTEM_FAILURE_EXIT_CODE:-1}" ERR
-  local base_image
-  base_image="$(compgen -G "${libvirt_default_pool_path}/runner-base-*.qcow2" | sort -n -t - -k3,3 | tail -n 1)"
 
-  if [[ -z ${base_image} ]]; then
-    echo 'Base image not found...'
+  if [[ ! -f /usr/local/lib/libvirt-executor/backing-vol-name ]]; then
+    echo 'Backing volume not found...'
     exit "${SYSTEM_FAILURE_EXIT_CODE:-1}"
   fi
+  local backing_volume
+  backing_volume="$(</usr/local/lib/libvirt-executor/backing-vol-name)"
 
-  qemu-img create -f qcow2 -b "${base_image}" -F qcow2 "${libvirt_default_pool_path}/$(vm_name).qcow2"
+  virsh vol-create-as "${libvirt_pool}" "$(vm_name).qcow2" 0 --format qcow2 --backing-vol "${backing_volume}" --backing-vol-format qcow2
   virsh define <(sed "s/\$vm_name/$(vm_name)/" /usr/local/lib/libvirt-executor/domain_template.xml)
   virsh start "$(vm_name)"
 
@@ -81,8 +81,7 @@ run() {
 # https://docs.gitlab.com/runner/executors/custom.html#cleanup
 cleanup() {
   virsh destroy "$(vm_name)" || true
-  rm "${libvirt_default_pool_path}/$(vm_name).qcow2"
-  virsh undefine "$(vm_name)"
+  virsh undefine --remove-all-storage "$(vm_name)"
 }
 
 case "${1:-}" in