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/...

Imported from xcp-rrdd.git b3c9de58bea50bd0ef94bf8ca5ba39ca99211200

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Imported-by: John Else <john.else@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.

Imported from xcp-rrdd.git b77f3f5352a71f16a3d7cc031b41e4325e1fe302

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Imported-by: John Else <john.else@citrix.com>
Merge pull request #1700 from johnelse/cp-4056-xs64bit

Update where rrdd looks in sysfs for VBDs