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: install libvirt and needed optional dependencies
   pacman: name=libvirt,virt-install,qemu-headless,dnsmasq,iptables-nft state=present
-  notify: reload firewalld
-
-- name: autostart default network on boot
-  command: virsh net-autostart default
   register: result
-  args:
-    creates: /etc/libvirt/qemu/networks/autostart/default.xml
+  notify: reload firewalld
 
-- name: start default network
-  command: virsh net-start default
+- 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