webadmin: Use new QoS ID

When creating a new QoS entity, commit the entity's ID in the engine
object representing it (so that it can be used e.g. to attach that QoS
entity to a network without retrieving from the DB).

Change-Id: I751e4cbd79c8c212531a2f5cd4b5ac0715319ce3
Bug-Url: https://bugzilla.redhat.com/1054311
Signed-off-by: Lior Vernia <lvernia@redhat.com>