libvirtd_start: Add case for only stop iptables

The new started libvirtd subprocess will lack permission on iptables
operation as not with selinux type 'virt_t', the iptables error in
libvirtd log is expected if found.

Signed-off-by: Wayne Sun <gsun@redhat.com>
We might need remove `/tmp/iptables.save` after test.