This patch introduces new xml elements under <blkiotune>,
we use these new elements to setup the throttle blkio
cgroup for domain. The new blkiotune node looks like this:

<blkiotune>
  <device>
    <path>/path/to/block</path>
    <weight>1000</weight>
    <read_iops_sec>10000</read_iops_sec>
    <write_iops_sec>10000</write_iops_sec>
    <read_bytes_sec>1000000</read_bytes_sec>
    <write_bytes_sec>1000000</write_bytes_sec>
  </device>
</blkiotune>
Signed-off-by: NGuan Qiang <hzguanqiang@corp.netease.com>
Signed-off-by: NGao feng <gaofeng@cn.fujitsu.com>