Report an error when the user tries to add an unknown attribute
instead of silently accepting (and ignoring) it.

Note that this commit also changes the behavior of public API
nm_utils_sriov_vf_from_str() to return an error when an unknown
attribute is found. I think the previous behavior was buggy as wrong
attributes were simply ignored without any way for the user to know.

Fixes: a9b4532f