The GetVolumeGroupSnapshotByID function makes it possible to get a
VolumeGroupSnapshot object from the Manager by passing a request-id.
This makes it simple for the Group Controller Server to check if a
VolumeGroupSnapshot already exists, so it is not needed to try and
re-create an existing one.
Signed-off-by: Niels de Vos <ndevos@ibm.com>