engine: Guard against old VDSM sending QoS as string

When no QoS was defined on a host, old versions (between 3.3 and 3.4)
of VDSM would send empty strings in the inbound/outbound entries,
where in fact they should have sent integers (and in the case of no
QoS no entries at all). This guards against such entries.

Change-Id: I8b68989bd6e8ca1ebf6e2f23e8bea3769f4ce5a9
Signed-off-by: Lior Vernia <lvernia@redhat.com>