virsh: add setting throttle blkio cgroup option to blkiotune

virsh: add setting throttle blkio cgroup option to blkiotune

With this patch, user can setup the throttle blkio cgorup
for domain through the virsh cmd, such as:

virsh blkiotune domain1 --device-read-bytes-sec /dev/sda1,1000000,/dev/sda2,2000000
--device-write-bytes-sec /dev/sda1,1000000 --device-read-iops-sec /dev/sda1,10000
--device-write-iops-sec /dev/sda1,10000,/dev/sda2,0

This patch also add manpage for these new options.

Signed-off-by: Guan Qiang <hzguanqiang@corp.netease.com>
Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>