Comment 5 for bug 1644318

Setting both hard / soft limits to 32768 should be enough [1]
Setting soft > hard have no sence since as soon as hard limit is reached you get error.
Setting soft < hard is also useless because we don't bother about quota management here.

To do so for a libvirt controlled by upstart:
* add to /etc/init/libvirtd.conf
  limit nofile 32768 32768
* restart libvirt service
* stop and start every VM to apply limits changes

[1] https://bugs.launchpad.net/fuel/+bug/1642941