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

@@ -2,9 +2,10 @@
 set -o nounset -o errexit -o pipefail
 readonly MIRROR="https://mirror.pkgbuild.com"
 readonly LIBVIRT_DEFAULT_POOL_PATH="/var/lib/libvirt/images"
+readonly STATE_DIR="/usr/local/lib/libvirt-executor"
 
 ssh() {
-    command ssh -i foo_rsa -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=off "root@${vm_ip}"  "${@}"
+    command ssh -i "${STATE_DIR}/id_rsa" -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=off "root@${vm_ip}"  "${@}"
 }
 
 get_vm_ip() {
@@ -65,7 +66,7 @@ create_vm_template() {
     qemu-img resize "${image_path}" 10G
     trap "virsh destroy ${vm_name}; virsh undefine ${vm_name} --remove-all-storage; exit 1" EXIT
     virt-install --name "${vm_name}" \
-                 --cloud-init user-data=$PWD/user-data \
+                 --cloud-init "user-data=${STATE_DIR}/user-data" \
                  --disk path="${image_path}",device=disk \
                  --os-type Linux \
                  --os-variant archlinux \