vdsm: Add ioTune support at vm creation

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

e.g.
Adding 'ioTune':{'read_bytes_sec': 6120000, 'total_iops_sec': 800} in specParams 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>

These parameters constrains the I/O bandwith for the device.

Change-Id: Ie4034648620ed9212c06c12607bb889d97cca9d6
Signed-off-by: MeiLiu <liumbj@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/14636
Reviewed-by: Dan Kenigsberg <danken@redhat.com>