common: introduce qos package, and storage qos

- Base Qos abstract class, in the future all QoS will inherit it.
- QosType (version >= 3.3  network, version >= 3.5 disk, cpu), to distinguish
between the types.
- StorageQos object as the first implementation.
- Handle field validation propeties.

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

Change-Id: I1a9af59277b5055453159f002f19046c0051d8ff
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>