Local unserved licenses cannot be used in Citrix or other virtual environments. Think about it - a company could get away with buying one license, then setting up 100 developers each using their own VM. For that reason, you must use the served license model when setting up PB on a VM.
And every developer that uses that VM must have a separate served license. If they also use a local unserved copy of PB, then that's two paid licenses for every developer. Those are the rules...
What we did at my company was setup a dedicated physical build machine - not a VM, but a physical box using a local unserved license. It just sat in a corner, and was not used for active development. Whenever a developer needed to kick off a build, they used a tool like Join.Me or TeamViewer to remotely view the desktop. Unfortunately, this forces the build machine to be single-threaded (only one build going at a time), but it is in license compliance.
-Paul Horan-