virISCSIDirectRefreshVol: Don't clear volumes in each run
    
    When fetching LUNs from iscsi server the
    virISCSIDirectReportLuns() is called. This function does some
    libiscsi calls and then calls virISCSIDirectRefreshVol() over
    each LUN found. It's unfortunate that the latter calls
    virStoragePoolObjClearVols() as we lose all LUNs processed
    in previous iterations.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Jiri Denemark <jdenemar@redhat.com>