Bridges without parents are not modeled after rack controller registers

I set up a test system with the following bridge configuration:

# brctl show
bridge name bridge id STP enabled interfaces
br0 8000.525400130e6f no ens3
br1 8000.000000000000 no
virbr0 8000.5254005caca1 yes virbr0-nic

As you can see, br0 is the only bridge interface linked to a physical interface. It is also the only bridge interface that appears under the controller in MAAS after the rack registers.

Now that LXD does not depend on lxcbr0, it would be good if rack registration considers all bridges, not just bridges linked only to a physical interface.