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>
Does this mean the data in the shared file is little-endian? If so, why not use Bitstring.extract_int64_le_unsigned?