core: simplify VDSGroup.equals()

Instead of manually doing a null-safe comparison for each VDSGroup
object, use ObjectUtils.objectsEqual().  Also, optimize the order of
comparison by looking at the id field first.

Change-Id: Iecd1e2314d2791764a4c3456fe21fc7931fe224d
Signed-off-by: Greg Padgett <gpadgett@redhat.com>