Add packages and support for Parallel Guest Tools

This adds complete support for using Parallel Guest Tools on NixOS. I have written this half-blindly, testing via ssh on the virtual machine on Mac OS X. A co-worker reported that everything works completely with it, including shared folders, profiles, shared mouse, 64- and 32-bit OpenGL acceleration, Guest Applications and shared clipboard. I've also run console utilities such as prlhosttime -- worked fine. He did not tested shared printers and snapshots -- they both at least start without any errors.

P.S.: I declare this the most awful proprietary package that I've seen or created, ever. I mean, just look at it! It had 20+ .sh and .py installer scripts, was inconsistent to the point of perfection, used 3 different ways to store to-be installed binaries and 4 ways to run them -- not talking about replacing system libraries, using dynamic libraries load, a daemon in bash, another one which just does hangs up and a whole lot of other surprises. I've not even mentioned half of its nasties!

P.P.S.: Because I don't have Parallels myself (nor a Mac), I won't be providing updates for this unless someone asks me to update it and provides a machine for testing.