core: Cluster parameters override - OVF

This feature allows to configure the 'emulated machine' and 'cpu model'
parameters for each VM separately instead of relying on the cluster default.

Change-Id: Ib772f8674ffb5297fbdce25f2fcf1121851a9def
Bug-Url: https://bugzilla.redhat.com/838487
Feature-page: http://www.ovirt.org/Features/Cluster_parameters_override
Signed-off-by: Eldan Shachar <eshachar@redhat.com>