virttest.utils_net: fixed clean IPv6 envrionment with error path

In order to avoid remote clean has effect on local then split local and
remote method to get IPv6 address list, and make sure the session is
alive w/o leaks.

Signed-off-by: Alex Jia <chuanchang.jia@gmail.com>
Merge pull request #1767 from chuanchang/fixed_cleanup_with_error_path_on_IPv6_setup

virttest.utils_net: fixed clean IPv6 envrionment with error path