The real NetworkManager service has a clear understanding how a valid
connection looks like. This is what nm_connection_verify() returns.

Let also our stub-service verify connections the same way.

Note that this is cumbersome, because the stub service uses python's
dbus module, while libnm only accepts creating NMConnection instances
from GVariant. Thus, we need to a cumbersome conversion first.

It would be better if test-networkmanager-service.py would also expose
normalized connections on D-Bus. But that requires the inverse converion
from GVariant to python dbus.