Configuring a VM with OS and Apps

Hi to everyone

I've read hundreds of pages in many documents and books about virtualizing Exchange 2010

They all discuss sizing for Exchange but there is no mention of the fact that it will sit on an OS that must first be sized.

When setting up a VM with WinSrv2008R2 for example do I just add what Exchange will need to what the server will need?

And once the VM is setup with Srv08 and Exch how is it that x amount of CPU and memory is allotted to Exchange alone?

It this something done in the server OS or the VM?

Funny this is never mentioned anywhere. Its like Exchange exists alone in a VM.

Thanks in advance.