gitlab_runner: Enable free-page-reporting for VMs

From the kernel patch series[1]:
"This series provides an asynchronous means of reporting free guest
pages to a hypervisor so that the memory associated with those pages can
be dropped and reused by other processes and/or guests on the host.
Using this it is possible to avoid unnecessary I/O to disk and greatly
improve performance in the case of memory overcommit on the host."[1]

The runner hosts may be memory overcommitted if there is too many
running VMs and containers at the same time, which this should help to
avoid.

[1] https://lore.kernel.org/linux-mm/20200211224416.29318.44077.stgit@localhost.localdomain/

roles/gitlab_runner/templates/domain_template.xml.j2

@@ -37,6 +37,7 @@
       <model type='virtio'/>
       <filterref filter='clean-traffic'/>
     </interface>
+    <memballoon model='virtio' freePageReporting='on'/>
     <rng model='virtio'>
       <backend model='random'>/dev/urandom</backend>
     </rng>