CA-136792: Create IO metrics from '/dev/shm' and sysfs.

With blktap3, blkback is not on the datapath except for rare occasions.
The statistics originally produced by blkback are now done from tapdisk.
This patch attempts to acquire statistics from under '/dev/shm'
and sysfs to generate only valid datasources.

In case we obtain stats from both sysfs and '/dev/shm':
- We take the maximum value for usecs from amongst sysfs and shm
- We average the values for the read and write metrics

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Merge pull request #1818 from siddharthv/blkback_stats_xs64

CA-136792: Read VBD IO metrics from '/dev/shm'.