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 create the datasources.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Revised-by: Zheng Li <zheng.li3@citrix.com>
While I think it's fine to copy the field names from blktap, it would be good to add a comment for each one describing what it is.