It needs to be possible to deserialize a connection hash into an
invalid NMConnection; in particular, AddAndActivateConnection()
explicitly allows this.

Previously, the SetFunc and NotSetFunc passed to
_nm_setting_class_override_property() could return a verification
error immediately, but this functionality has to go away if we're
going to be able to deserialize invalid connections.

That functionality was only used in the handling of invalid virtual
interface names; reorganize how that code works so that
NMSettingConnection does all of the verification itself. (The code to
make sure that it returned the "correct" error domain in that case
turned out to be irrelevant, since the setting error domains don't get
serialized over D-Bus correctly anyway.)