Intel AMT Power Address: assign dynamically

When configuring a machine with power type Intel AMT, the power address is required. I think this should be automatically filled-in (and maintained!) from an IP assignment that MAAS already made to the machine.
Currently, if/when the IP assignment for the machine changes, the power assignment breaks and requires manual intervention.

This could apply to other power types, too. Basically, the Power Address could be a reference to an interface on the machine, instead of a literal IP.