Fix #1223: Non existent tango devices considered Undefined instead of NotReady

The current behavior considers a non existent Tango Device as a NotReady device, and devices defined in db that are off or in unknown state as Undefined.
This MR inverts that behavior: non existent devices are Undefined and existent devices that are off or in a not ready state (init, fault, disabled or unknown) are NotReady.
This new behavior was discussed in #1223, considering the previous one as a bug.