The system is keying off of the Inv_AeX_VirtualMachine table. When hardware inventory runs it is able to determine which machines are virtualized. Once that information is saved in the database a scheduled task that run nightly will convert the computer resource to a virtual machine resource.
The following SQL should provide the collection you are after.
SELECT _ResourceGuid AS 'Guid' FROM Inv_AeX_VirtualMachine
HTH