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,virt-install,cdrtools,qemu-headless,dnsmasq,iptables-nft state=present
+  pacman: name=libvirt,qemu-base,dnsmasq,iptables-nft state=present
   register: result
 
 - name: reload firewalld