[PATCH] Add in measuring of Plug and Unplug traces.

[PATCH] Add in measuring of Plug and Unplug traces.

Added in the display of plug information on a per-device basis.

% Time Q Plugged: Percent of the total time during which the queue
                  was plugged

==================== Plug Information ====================

       DEV |    # Plugs # Timer Us  | % Time Q Plugged
---------- | ---------- ----------  | ----------------
 ( 68, 64) |        833(         0) |   0.36%

Signed-off-by: Alan D. Brunelle <Alan.Brunelle@hp.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>