core: Cluster parameters override - fields\db

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: I64d829aa6a68b1c97fb59d2a7a21351a00c92c40
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>