I agree as well. Likely the OP has multiple datastores in which case a datastore cluster with storage IO like sjesse mentioned would work.
One item though, this is an unusual request. Typically, VDI doesn't need to have this limit in place as dedicated storage is in place. This could be an over subscription issue on the storage I/O or perhaps a specific app generating the I/O. What is the behavior you are seeing that requires limiting the I/O?
