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/libvirt/tasks/main.yml

+---
+- name: remove iptables to solve iptables<->iptables-nft conflict
+  pacman: name=iptables force=yes state=absent
+
+- name: install libvirt and needed optional dependencies
+  pacman: name=libvirt,qemu-base,dnsmasq,iptables-nft state=present
+  register: result
+
+- name: reload firewalld
+  service: name=firewalld state=reloaded
+  when: result.changed
+
+- name: autostart default network on boot
+  file: src=/etc/libvirt/qemu/networks/default.xml dest=/etc/libvirt/qemu/networks/autostart/default.xml state=link owner=root group=root
+
+- name: start and enable libvirtd
+  systemd: name=libvirtd enabled=yes state=started daemon_reload=yes