The NewSnapshotByID() function makes it possible to clone a new Snapshot
from an existing RBD-image and the ID of an RBD-snapshot on that image.
This will be used by the VolumeGroupSnapshot feature, where the ID of an
RBD-snapshot is obtained for the RBD-snapshot on the RBD-images.
Signed-off-by: Niels de Vos <ndevos@ibm.com>