When I wrote the head to head review of Parallels and Fusion for InfoWorld, I also did a sidebar on remote control of hypervisors and guests.
At the time I wasn't aware that Parallels also has a command line tool called prlctl for managing the hypervisor and controlling guests. A simple "man prlctl" told me all I needed to know and a minute later, I was starting, stopping, and suspending guests from the terminal. The screenshot to the right (click to enlarge) shows it in the process of suspending a Vista operating system guest on my machine.
In addition to power-on settings, you can create, clone, delete, and perform other operations on guests, manage snapshots, and change guest settings. The tool can work remotely if needed--I just used it on my local machine. An API is available if you want to write your own tools to interface with Parallels.
There are a lot of little known power-user features in both Parallels and Fusion. Partly that's because both companies have targeted Windows users as the primary customer and thus highlight features that they'll appreciate.