engine: Serialize null QoS to empty dictionary entry

Previously it was serialized to no dictionary entry at all, in which
case the VDSM would leave the QoS settings as they already were
(instead of overwriting them to be unlimited).

Change-Id: I6a1316f81a7b89783eae04c055e2e686df5e7c98
Bug-Url: https://bugzilla.redhat.com/1077684
Signed-off-by: Lior Vernia <lvernia@redhat.com>