engine: Persist empty QoS for VM

This has been fixed previously for host QoS but not for VMs; in case
no QoS is assigned to a profile, an empty entry is sent (rather than
no entry).

Change-Id: Iffe16ac863d338aa82cab1248b8fb1769ac7fcd3
Bug-Url: https://bugzilla.redhat.com/1077684
Signed-off-by: Lior Vernia <lvernia@redhat.com>