Sorry to have misread your post. I read “…how are clusters configured in vCloud.” as in adding cells in VCD. Hence the KB!!
Let me think about your question and your further explanation. If you want to "share disk at the machine level, not the OS," would it not be easy to provision storage through SAN, though I don't recall giving shared storage via SAN? It has been a while that I have done SAN administration though, to look into that for you. Kindly explore your storage option through your storage device.
Now going back to the Exchange Cluster question; DAG simplifies all of that. I would assign distinct storage to each VM and let DAG do the replication and HA of your data base. Seems would be a better design.
I hope this helps.