tool.nwfilter: do not close a connection twice

My commit eb2ef55534 caused all libvirtconnections to be closed when the
calling process exists. As reported by Nir on
http://gerrit.ovirt.org/27313 that commit has a bug and was sloppily
verified. As of eb2ef55534, no one outside the libvirtconnection module
should close a libvirt connection.

Change-Id: I9dfe4b98401fb2c352f87952e182fb8df06fd0e3
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27315
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>