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

@@ -101,7 +101,7 @@ prepare() {
         echo "Error no VM template found"
         exit 1
     fi
-    vm_name="foo"
+    vm_name="$(get_vm_name)"
     if ! virt-clone -o "${vm_template}" -n "${vm_name}" --auto-clone --reflink; then
         virt-clone -o "${vm_template}" -n "${vm_name}" --auto-clone
     fi
@@ -111,14 +111,14 @@ prepare() {
 
 # https://docs.gitlab.com/runner/executors/custom.html#run
 run() {
-    vm_name="foo"
+    vm_name="$(get_vm_name)"
     wait_for_ssh "${vm_name}"
     ssh bash < "${1}" || exit "${BUILD_FAILURE_EXIT_CODE:-1}"
 }
 
 # https://docs.gitlab.com/runner/executors/custom.html#cleanup
 cleanup() {
-    vm_name="foo"
+    vm_name="$(get_vm_name)"
     virsh destroy "${vm_name}" || true
     virsh undefine "${vm_name}" --remove-all-storage
 }