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

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 first attempts to read the statistics from under '/dev/shm'
and sysfs to generate the required datasources.

- 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>