ethdev: fix iterator default behaviour for representors

ethdev: fix iterator default behaviour for representors

The iterator was matching all representors if it was not specified
in the devargs string. It was a wrong default behaviour.

If there is no representor parameter in the devargs, the iterator
should not match any representor port.

The implementation of the default behaviour would be simpler
if a "no match" handler is added to rte_kvargs_process().
As it requires an API breakage, it will be reworked later.

Fixes: a7d3c6271d55 ("ethdev: support representor id as iterator filter")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>