core: removal of unneeded callback passing

The callback can be executed from sp.py after completing the operation,
eliminating the need to pass it to imageSharing.

Change-Id: I5a6e455dc3824f695fab241197ac628713f683a9
Signed-off-by: Liron Aravot <laravot@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26760
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Yoav Kleinberger <ykleinbe@redhat.com>
Reviewed-by: Xavi Francisco <xfrancis@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>