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

@@ -46,16 +46,6 @@ wait_for_ssh() {
   exit "${SYSTEM_FAILURE_EXIT_CODE:-1}"
 }
 
-wait_for_vm_shutdown() {
-  for _ in {1..10}; do
-    if LC_ALL=C.UTF-8 virsh domstate "${1}" | grep -F "shut off"; then
-      return 0
-    fi
-    sleep 1
-  done
-  return 1
-}
-
 # https://docs.gitlab.com/runner/executors/custom.html#prepare
 prepare() {
   # shellcheck disable=SC2064