While installing XenDesktop 7 VDA, there is an option to specify the Controllers later. If you choose this option, you are told that you can specify the controllers later, but it isn't clearly defined where to do this. It's technically in the eDocs, but it was much easier to find after I knew what I was looking for.
You can specify the controllers, and make them register, without restarting the machine. It is a two step process:
Step 1: Create the list of DDCs in the registry
Start by editing the following registry key. It should contain a list of your DDCs separated by a space (not a comma like some older Citrix articles show). If you use a comma, the process will break, so make sure it's a space.
Step 2: Restart the Citrix Desktop Service
This is pretty self explanatory, but if you're looking for a command line to get the job done, the following will work for you:
Net Stop BrokerAgent & Net Start BrokerAgent
Once the Citrix Desktop Service comes online, the VDA will register with each of the controllers that you specified in the registry key above. It may take a few minutes, depending on the number of controllers you have, so be patient.