CA-147261: Add average queue size metric for tapdisk3

Prior to this patch, the average queue size metric was missing.
This patch computes this metric using the stats exposed by tapdisk3.
Average queue size is defined as the sum of the average read and write queues.
Additionally, we maintain two variables to retain the previous values of
this metric.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Oh, I missed out on the code below these comments. For some reason (I probably missed it -- maybe something funny when the page loaded) I only saw the code above John Else's comments. Yes, summing them up should do the trick and the patch looks good to me (bare in mind I don't do ocaml). To verify the results, I recommend running a steady workload (e.g. seq reads with io depth=1 on a raw vdi) on a guest with PV drivers and comparing the output of this tool with that of xsiostat.