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 via C stubs to create the datasources.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Revised-by: Zheng Li <zheng.li3@citrix.com>
CA-136792: Add ocamldoc style comments for the blktap3_stats record.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>