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>
IIUC this file is also going to be used by xsiostat. Ultimately we might have to end up reversing it in one place or another.