BibIndex: fix for virtual index filtering

* Fixes function filter_for_virtual_indexes

Reported-by: Henning Hraban Ramm <invenioml@fiee.net>
Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>