tests: add test cases for netfilter object Using qtest qmp interface to implement following cases: 1) add/remove netfilter 2) add a netfilter then delete the netdev 3) add/remove more than one netfilters 4) add more than one netfilters and then delete the netdev Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com> Signed-off-by: Jason Wang <jasowang@redhat.com>