Add more control on VM power states

[michaelsanford]

The user should be allowed more control over power states. I propose exporting these as well, for flexibility:

• poweroff
• acpipowerbutton
• acpisleepbutton

(Notably, it is not currently possible to power off a virtual machine.)

In addition to the currently-supported:

• savestate (currently == "stop")
• resume
• pause
• reset