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