When the rbd.Manager creates a VolumeGroupSnapshot, each RBD-snapshot
that is created as part of the RBD-group needs to be cloned into its own
RBD-image that will be used as a CSI Snapshot.
The VolumeGroup.CreateSnapshots() creates the RBD-group snapshot and
returns a list of the Snapshot structs.
Signed-off-by: Niels de Vos <ndevos@ibm.com>