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

Signed-off-by: Alex Jia <chuanchang.jia@gmail.com>
Merge pull request #1716 from chuanchang/support_setup_ipv6_on_local_and_remote

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