I have had customers simulate I/O with the sio tool on the now site, in the toolchest. In one particular environment they wrote a autologin script for each VM (meaning that when the desktop booted, a simulated user auto logged in). Within the users login script it called the sio tool to begin I/O generatation for a defined period of time, pause, and then initiate I/O again.
The customer would basically power on virtual desktops and the script would be called as the simulated I/O user callled on the script. The testing worked very well. The thing I like about the SIO tool is that it is very simple to understand and very easy to call upon in a logon script.