Ignore pre-existing CSI volumes known to SLRP.

A pre-existing CSI volume can be known to an SLRP if there is a CSI volume state checkpoint for the volume, but the corresponding resource checkpoint is missing. This typically happen when the agent ID changes which would make the SLRP lose all its resource provider state. In such a case, we should not treat these volumes as unmanaged pre-existing volumes (i.e., disk resources without profiles). For now we should not report them, and consider designing a way to recover these volumes.