[BLKBACK] Output statistics to sysfs.

[BLKBACK] Output statistics to sysfs.

The location is 'statistics' directory under each VBD directory, for
example, /sys/devices/xen-backend/vbd-x-xxxx/statistics.

The 'statistics' directory includes the following three statistical
information:
  oo_req
       is the number of requests held up due to full pipeline.
  rd_req
       is the number of processed READ requests.
  wr_req
       is the number of processed WRITE requests.

Signed-off-by: Satoshi UCHIDA <s-uchida@ap.jp.nec.com>