I have also failed to find the documentation for this. I have put together a list of the states I have found using vCO:
Approved
AwaitingApproval
BuildingMachine
Complete
DeactivateMachine
Disposing
Expired
Finalized
Finished
InstallTools
Leased
MachineActivated
MachineProvisioned
Off
On
Pending
Provisioning
Rebooting
RegisterMachine
Rejected
Requested
Resetting
Resubmitted
ShuttingDown
Suspending
TurningOff
TurningOn
UnprovisionMachine
Waiting
WaitingToBuild
I have not tested this so feedback is welcome.