CP-4056: Update where rrdd looks in sysfs for VBDs

With the move to Linux 3.x kernel the paths that were:

    /sys/device/xen-backend/vbd-xx-xxxxx/...

have now moved up one directory to

    /sys/device/vbd-xx-xxxxx/...

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Read write-related stats from correct file

The metrics wr_reqs, wr_avg_usecs and wr_max_usecs were being read from the
rd_usecs file and not the wr_usecs file.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #7 from simonjbeaumont/cp-4056

Update where rrdd looks in sysfs for VBDs