NMManager was failing to initialize if there was only a private bus,
despite the fact that this is exactly the use case that the private
bus was added for.

The only other potentially-failing code in nm_manager_new() was adding
prop_filter to the D-Bus connection, but this can't really fail, so
just assert that it doesn't. And now, nm_manager_new() always
succeeds, so update the caller for that.