spec: don't touch existing nwfilters on update

spec: don't touch existing nwfilters on update

Nwfilter can be edited by the user and we don't want to overwrite the editings.
Also the filters in %{datadir} does not have UUIDs and these are generated on
libvirtd start. Thus this patch also fixes regeneration of UUIDs on libvirtd
update.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>