vsphere_guest: Add reboot and shutdown to powerstate (requires tools).

Added two additional power states "rebooted" and "shutdown."

The difference between those two and "restarted" and "powered off" is that an actual shutdown signal is sent to the guest OS, as opposed to the equivalent of yanking out the power cord. This is useful is you are reconfiguring a VM and need it shutdown for said reconfiguration, but don't want to risk the integrity of the guest OS.

The two options fail if the machine is powered on and tools is not running.