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>