CA-133656: iops is always (nearly) zero

Problem: The iops values are always or at least nearly zero

Root Cause: rrdd_plugin_iostat query those values from /sys/block/tdX/stat.
It treated the 1st (zero-based) and 5th columns as the the io-read-requests
and io-write-requests values. However, according to
https://www.kernel.org/doc/Documentation/block/stat.txt, 1st/5th are
io-read-merged and io-write-merged and 0th/4th are io-read-requests and
io-write-requests.

Solution: Revise the indices.

Signed-off-by: Kaifeng Zhu <kaifeng.zhu@citrix.com>
Merge pull request #7 from kaifengz/CA-133656

CA-133656: iops is always (nearly) zero
rrdp_iostat: push the exception handler for listing a domain's VBDs inwards

No functional change; refactoring only.
CA-141867: Look in both /local/domain/0/backend/vbd and vbd3 when listing a domain's VBDs

tapdisk2 uses vbd, tapdisk3 uses vbd3, so the domain's VBDs could be in either.

(We might have looked up the correct backend path for each VBD from the frontend
info, but we can't trust that a malicious frontend might not manipulate that
info.)
Merge pull request #10 from jjd27/CA-141867-v2

CA-141867 (v2): backend paths may be either /local/domain/0/device/vbd or vbd3