engine: Make VDSPartial, a partially loaded VDS object

Since loading a full VDS object is time consuming, this patch introduces
a VDSPartial object, that is much faster to load. VDSPartial loading takes
5ms as opposed to 8ms for VDS

Change-Id: Ie2401686d43daa6bcfb810b13aea6cc2570e5520
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=992883
Signed-off-by: lzelkha@redhat.com <liran.zelkha@gmail.com>