virISCSIDirectRefreshVol: Don't clear volumes in each run

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>