CA-136972: Read raw stats from blktap3

With blktap3, the blkback stats structure is maintained on a shared page
This patch fetches the required counters to seeking to the appropriate
locations.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Actually no ignore that - it wouldn't be equivalent. But, since we're the only customer using this file, can't we coordinate so that it's in a format where we can read it without the copy-and-reverse?