spec: Fix permissions of nwfilter XMLs
    
    The nwfilter XMLs in /etc are defined as %ghost in the spec file, which
    means rpm will not install them, but it will record its existence and
    permissions in the database. During installation the files are copied in
    a %post scriptlet from /usr/share/libvirt/nwfilter, but once libvirtd is
    restarted, it will rewrite the files to add generated UUIDs.
    
    While RPM recorded 644 mode for the XMLs, libvirt saves them with 600
    and thus any future attempt to verify the libvirt-daemon-config-nwfilter
    package would fail. We need to tell RPM the ghost files are supposed to
    have 600 permissions.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1559284
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Erik Skultety <eskultet@redhat.com>