Hi Peer,
Since TestComplete v10 or so, the license server can be installed on the servers that host virtual machines. License server is small and quite reliable (at least, I did not hear about problems caused by it), so this may simplify the task of finding the physical box for it.
As for its setup on the virtual box, as it bounds the license to the physical hardware, I am not sure that it will be possible in the nearest future to install license server on the virtual machines.