This is an extension of the problem reported in bug 613942
We're responding to a steady stream of tickets about "failed" installs on the "guest-xx" systems. This is obviously because those systems can't be directly installed from a simple workflow. As such, it doesn't make sense for them to be offered up in the list of Available systems for a reserve workflow. They really should be well hidden from mere mortals.
Here's the latest ticket. I can find several more if you want.