driver/powerdriver: raise NotImplementedError on PDUDaemonDriver.get()

pdudaemon does not implement port state retrieval. PDUDaemonDriver.get()
returns None. This leads to cases where None is interpreted as "off",
i.e. with constructs relying on `bool(drv.get())`. `labgrid-client power
get` is one of those cases.

So raise a NotImplementedError instead.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

labgrid/driver/powerdriver.py

@@ -411,4 +411,4 @@ def cycle(self):
 
     @Driver.check_active
     def get(self):
-        return None
+        raise NotImplementedError("pdudaemon does not support retrieving the port's state")