Revert "engine: Make VDSPartial, a partially loaded VDS object"

This reverts commit dfedf74bd738a568418b4262dd5225fabc0d1083.

Change-Id: I6bf4dbb3138d33b337c0f530b15fe23dfad998b5
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>