sp: improve variables in _linkStorageDomain

In order to avoid future confusion on _linkStorageDomain variables,
they have been renamed with more meaningful names.

Related-To: https://bugzilla.redhat.com/show_bug.cgi?id=1116585
Change-Id: I696529865d270318662233f5c719f30d5b85d112
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31090
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>