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>
The close should really be in a `finally`-type clause, otherwise we'll risk leaking FDs if any exception is thrown.