netdev: Use virNetDevIsVirtualFunction() properly

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.