ixgbevf: Add RSS Key query code

Add the ixgbevf_get_rss_key() function that queries the PF for an RSS
Random Key using a new VF-PF channel IXGBE_VF_GET_RSS_KEY command.

This patch adds the support for 82599 and x540 devices only. Support for
other devices will be added later.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>