storageServer: Improve iscsi connection logging

We have several bugs where iscsi storage connection are not disconnected
properly, but the logs contains no info that can help debugging such
issue. Now it should be very clear why disconnect operation is skipped.

Change-Id: I9de4ec72f09777e84273933ff41e9d1424e7aa09
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/36028
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>