ethdev: allow to get RSS hash functions and key

ethdev: allow to get RSS hash functions and key

1) Add a new function "rss_hash_conf_get" in the PMD API to retrieve the
   current configuration of the RSS functions and/or of the RSS key used
   by a NIC to compute the RSS hash of input packets.
   The new function uses the existing data structure "rte_eth_rss_conf" for
   returning the RSS hash configuration.

2) Add the ixgbe-specific function "ixgbe_dev_rss_hash_conf_get" and the
   igb-specific function "eth_igb_rss_hash_conf_get" to retrieve the RSS
   hash configuration of ixgbe and igb controllers respectively.

3) Add the command "show port X rss-hash [key]" in the testpmd application
   to display the RSS hash configuration of port X.

Signed-off-by: Ivan Boule <ivan.boule@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>