Is possible to enable virtio-RNG or other entropy system?

Community Support
2

You can add features not supported by OpenNebula using the RAW parameter. For your case to add the virtio rgn example from libvirt documentation you can use this RAW parameter in the VM template:

RAW=[
  TYPE="kvm",
  DATA="<devices>
  <rng model='virtio'>
    <rate period="2000" bytes="1234"/>
    <backend model='random'>/dev/random</backend>
  </rng>
</devices>"]