generic.tests: fix nfs serivce not stop issue

NFS serivce listening on both ipv4 and ipv6 adress. And it mounted with
'localhost', both ipv4 and ipv6 connection setup. In script, only reject
IPv4 pack but ipv6 connection still working. So write error not be trige,
VM still runing, so verify VM 'paused' failed.
Fix this defeat by update iptables command and bind nfs server listen on host
IP.

Signed-off-by: Xu Tian <xutian@redhat.com>