Storage performance is the new bottleneck for VM performance. In VMware vSphere, you can actually limit disk I/O on a per VM basis. The setting can be found by powering off a VM, right-clicking the VM in the vSphere Client Inventory, selecting "Edit Settings". Click the Resources tab and then select Disk. This is where you will find the "Limit - IOPs" setting.
There is one caveat though, “combined total” only goes for the disks which are stored on the same datastore. So if you have 4 disks and they are stored across 4 datastores then each of the individual limits apply respectively.