ethdev: allow iterating with pure class filter

ethdev: allow iterating with pure class filter

If no rte_device is given in the iterator,
eth_dev_match() is looking at all ports without any restriction,
except the ethdev kvargs filter.

It allows to iterate with a devargs filter referencing only
some ethdev parameters. The format (from the new devargs syntax) is:
	class=eth,paramY=Y

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>