I think you are very close. You may want to verify the class you are targeting in the dynamic group you have created. You will want to target the "VMGuest Virtual Machine" class. An alternative method for targeting your virtual desktops, you may want to try keying off of "Resource Pool Name" or "Cluster Name" properties. This way, if you end up changing your VDI naming scheme down the road, you will not be impacted.
Let us know how it goes!