engine: Check VDSM supported clusters to deserialize QoS

It is not in fact correct check the cluster's compatibility version to
see if Host Network QoS is supported, since GetVdsCapabilities should
work even if the VDSM is too old for the cluster's compatibility
version, and in case there's QoS configured on a host we'd want it to
be reported even if it resides in an older cluster.

Change-Id: I8a5303222d51459019c1aaaf6754867cd234fe9d
Bug-Url: https://bugzilla.redhat.com/1056155
Signed-off-by: Lior Vernia <lvernia@redhat.com>