gitlab_runner: Add VM based executor (libvirt-executor)

For some workloads running in a container is too restrictive, ex:
arch-boxes (loop device, filesystem mount, pacstrap) and archiso
(pacstrap). Currently they both run a TCG accelerated QEMU VM, which is
very slow and painful to work with. We should provide a better option to
our users!

This adds a hardware accelerated VM for this kinds of workloads, which
is way faster and you can do whatever you like (mostly)!

Fix #283

---

TODO:

• Enable KSM?
• Get archiso to build
  • Done: https://gitlab.archlinux.org/klausenbusk/archiso/-/pipelines/10976
• Use BUILD_FAILURE_EXIT_CODE and SYSTEM_FAILURE_EXIT_CODE: https://docs.gitlab.com/runner/executors/custom.html#error-handling
• Fix existing users to use the secure-vm tag

roles/gitlab_runner/files/libvirt-executor

@@ -63,7 +63,6 @@ create_vm_template() {
     trap "rm -f ${image_path}" EXIT
     curl -sSf "${MIRROR}/images/v20210815.31636/Arch-Linux-x86_64-cloudimg-20210815.31636.qcow2" --output "${image_path}"
     qemu-img resize "${image_path}" 10G
-    # TODO remove exit 1 andre steder
     trap "virsh destroy ${vm_name}; virsh undefine ${vm_name} --remove-all-storage; exit 1" EXIT
     virt-install --name "${vm_name}" \
                  --cloud-init user-data=$PWD/user-data \
@@ -89,8 +88,6 @@ create_vm_template() {
     virsh shutdown "${vm_name}"
     wait_for_vm_shutdown "${vm_name}"
     virsh domrename "${vm_name}" "${vm_name%%_tmp}"
-    # TODO: delete machine-id
-    # 192.168.122.170
     trap - EXIT
 }