I/O rate limit for volume copy with dd

I/O rate limit for volume copy with dd

Currently, volume copy operations consumes disk I/O bandwidth heavily and
may slow down the other guest instances. This patch limits bandwidth for
volume copy to mitigate interference to other instance performance.

In this implementation, 'dd' is put in a blkio cgroup for throttling,
when CONF.volume_copy_bps_limit is set to non-zero.

Change-Id: I32347d2e28842725207a1b3afac7fe137a3fd3b4
Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Implements: blueprint limit-volume-copy-bandwidth