restapi: add QoS RESTful interface for storage QoS

- POST: /ovirt-engine/api/datacenters/{datacenter:id}/qoss; body: qos; response: qos
<qos type="storage">
  <name>test_qos</name>
  <description>aaaaa</description>
  <max_throughput>1</max_throughput>
  <max_read_throughput>1</max_read_throughput>
  <max_write_throughput>1</max_write_throughput>
  <max_iops>1</max_iops>
  <max_read_iops>1</max_read_iops>
  <max_write_iops>1</max_write_iops>
</qos>
- GET: /ovirt-engine/api/datacenters/{datacenter:id}/qoss; response: qoss
- DELETE: /ovirt-engine/api/datacenters/{datacenter:id}/qoss/{qos:id};
- GET: /ovirt-engine/api/datacenters/{datacenter:id}/qoss/{qos:id}; response: qos
<qos type="storage" href="/ovirt-engine/api/datacenters/00000002-0002-0002-0002-000000000321/qoss/a66577ff-d5f1-40f7-aebb-0b350ad8bb8c" id="a66577ff-d5f1-40f7-aebb-0b350ad8bb8c">
  <name>test_qos</name>
  <description>aaaaa</description>
  <data_center href="/ovirt-engine/api/datacenters/00000002-0002-0002-0002-000000000321" id="00000002-0002-0002-0002-000000000321"/>
  <max_throughput>1</max_throughput>
  <max_read_throughput>1</max_read_throughput>
  <max_write_throughput>1</max_write_throughput>
  <max_iops>1</max_iops>
  <max_read_iops>1</max_read_iops>
  <max_write_iops>1</max_write_iops>
</qos>
- PUT: /ovirt-engine/api/datacenters/{datacenter:id}/qoss/{qos:id}; body: qos; response: qos

For more information see: http://www.ovirt.org/Features/aggregate_QoS,
http://www.ovirt.org/Features/blkio-support#RESTful_API

Change-Id: I1a9af59277b50554531522802f19046c0051d82f
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>