Most important part is to use ISCSI protocol instead of shared virtualbox devices.

Get a box to act as ISCSI target, while your cluster nodes are initiators.
This is your disk subsystem from which your will make failover zpools or metasets.

Also the document is outdated for current releases, you might want to keep that in mind.

Once you have ISCSI setup done, you can just follow the regular documentation for your release keeping mind of iscsi notes if existing.

Hope that helps
Best regards
Peasant.