I/O rate limit for volume copy with qemu-img convert

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, 'qemu-img' is put in a blkio cgroup for throttling,
when CONF.volume_copy_bps_limit is set to non-zero.

Change-Id: I8403d4bcafc4279654034da97aa9b6c097f9e649
Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama@hds.com>
Implements: blueprint limit-volume-copy-bandwidth
Merge "I/O rate limit for volume copy with qemu-img convert"