add blkIoTune support for a specific device at vm creation

[Mei Liu]

Hi all,
I would like to add blkIoTune support for a specific device at vm creation.

The code parses the 'blkIoTune' descrption for block devices at vm creation
time and adds iotune tag accordingly.

e.g.
Adding 'blkIoTune':{'read_bytes_sec': 6120000, 'total_iops_sec': 800} for a
block device will add the following in dom xml for that device.

<iotune>
      <read_bytes_sec>6120000</read_bytes_sec>
      <total_iops_sec>800</total_iops_sec>
</iotune>

The patch is under review in http://gerrit.ovirt.org/#/c/14636/ .

Does the patch meet the requirement of the engine or the whole 
architecture? Are the new parameters properly placed?  Any suggestions 
are welcomed.

TIA.

Best reagrds,
Mei Liu (Rose)