block: Compute minimum, maximum and average I/O latencies

block: Compute minimum, maximum and average I/O latencies

This patch keeps track of the minimum, maximum and average latencies
of I/O operations during a certain interval of time.

The values are exposed in the BlockDeviceTimedStats structure.

An option to define the intervals to collect these statistics will be
added in a separate patch.

Signed-off-by: Alberto Garcia <[email protected]>
Message-id: c7382dc89622c64f918d09f32815827772628f8e.1446044837[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>