raw/ioat: create separate statistics structure

raw/ioat: create separate statistics structure

Rather than having the xstats as fields inside the main driver structure,
create a separate structure type for them.

As part of the change, when updating the stats functions referring to the
stats by the old path, we can simplify them to use the id to directly index
into the stats structure, making the code shorter and simpler.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: Kevin Laatz <kevin.laatz@intel.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>