ixgbevf: add support for X550 VFs

This patch adds initial support for VFs on a new mac - X550.

The patch adds the basic structures and device IDs for the X550 VFs
that would allow the driver to load and pass traffic.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>