netdev: Use virNetDevIsVirtualFunction() properly
    
    virNetDevIsVirtualFunction() returns 1 if the interface is a
    virtual function, 0 if it isn't and -1 on error. This means that,
    despite the name suggesting otherwise, using it as a predicate is
    not correct.
    
    Fix two callers that were doing so adding an explicit check on
    the return value.