storage_source: Add flag storing whether threshold event was registered with index

storage_source: Add flag storing whether threshold event was registered with index

When users register the threshold event for the top level image with an
explicit index (e.g. vda[3]) they are clearly expecting the index in the
event.

This flag will help avoiding emission of the second event without the
index when the client clearly requested one with the index.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>