virttest.utils_net: Add openflow manager API for ovs bridge

Add a method to manager openflow rules.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
qemu.test: add openflow test

This patch add general openflow rules set(drop, normal)
test for arp, icmp, ip, tcp and udp

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #1028 from humanux/openflow_v2

Openflow test