igbvf: convert to strongly typed descriptors

The igbvf driver for some reason never strongly typed it's descriptor
formats. Make this driver like the rest of the Intel drivers and use
__le* for our little endian descriptors.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>