Go to <yourvmid> --> Hardware --> your disk "Hard Disk (xxxx)" --> Double click. This "Edit: Hard Disk (scsi0)" dialog has TWO pages, the second reads:
Read limit /MB/s)
Write limit (MB/s)
Read limit (ops/s)
Write limit (ops/s)
That should be sufficient, if it works as I understand it.
Disclaimer: not tested...