`doSnapshotClone()` returns a new `rbdVolume` object from a temporary
snapshot. This conversion drops the `SourceVolumeID` attribute, as a
`rbdVolume` does not have that.
After converting the `rbdVolume` back to a `rbdSnapshot`, the
`SourceVolumeID` attribute can be set again, and the `ToCSI()` function
can create an appropriate CSI Snapshot struct.
Signed-off-by: Niels de Vos <ndevos@ibm.com>
(cherry picked from commit b3faa04504618fe5e50c5edf9ee368a545d2aa62)