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>
I've addressed this comment by using with_input_channel to prevent fd leaks :)