Hi
You must to use a Shared Datastore.
http://docs.opennebula.org/5.2/deployment/open_cloud_storage_setup/fs_ds.html#fs-ds
At /var/lib/one/datastores you must to link to your nfs mounted dir.
Example:
I mount my NFS export at /mnt/opennebulaA and create the images and system on it.
Then link to the /mnt/OpenNebula/system and /mnt/OpenNebula/images to the /var/lib/one/datastores/0 and /var/lib/one/datastores/1
I hope helps to you…