virttest.utils_net: support IPv6 setup and cleanup both local and remote...

With this patch, it will be very easy to setup and clean up static IPv6  test environment both
local and remote host.

The following is a basic usage:
from virttest.utils_net import IPv6Manager

1. setup
ipv6_obj = IPv6Manager(test_dict)
ipv6_obj.setup()

2. cleanup
if ipv6_obj:
    del ipv6_obj