Added InstanceID to CIM_StorageExtent.
Now both LMI_VGStoragePool and all CIM_StorageExtents have InstanceID
property to uniquely identify them. It will simplify the client scripts,
which now don't need to distinguish between CIM_StorageExtent.DeviceID and
LMI_VGStoragePool.InstanceID.
Still, CIM_StorageExtent.DeviceID is the primary key for StorageExtents.