A loopback interface cannot be attached to a controller interface (in kernel).
Also, we have special handling for the loopback address 127.0.0.1. It's
not clear how that should behave when the loopback device would be
attached to another interface.
Just reject such configuration as invalid.
Fixes: e8618f03d7 ('support loopback interface')
Support managing the loopback interface through NM as the users want to
set the proper mtu for loopback interface when forwarding the packets.
Additionally, the IP addresses, DNS, route and routing rules are also
allowed to configure for the loopback connection profiles.
https://bugzilla.redhat.com/show_bug.cgi?id=2060905