Virtual server is limited to one CPU per VM. When you go to Hyper-V, it’s posible to use up to 4 virtual CPU inside each VM.
To be able to use more than one CPU you have to change from uni-processor hal to multiprocessor hal. So far I could research this change is not well documented for Windows Server 2008. Here it is:
Execute MSConfig, and then go to tab Boot -> Advanced Options…, check option “Detect Hal”, restart to these options take effect.
Or using command line:
bcdedit /set detecthal yes