tool: Fix compatibility with Python 2.6

Commit 398d016b58 was merged too quickly, before Python developers could
add set literals to Python 2.6. Since this is quite late now to add it,
I think we should use the good old set() syntax.

Change-Id: I77c02b69eaf1852b227d04c194461e542f0a096a
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31734
Reviewed-by: Francesco Romani <fromani@redhat.com>
Tested-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>