virttest.utils_net: Add ipv6 support

This patch makes get_guest_ip_addr to support ipv6,
also adds some methods to get ipv6 neighbours info,
and check whether ipv6 neighbour is reachable.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
virttest.remote: Add ipv6 support

This patch makes remote login and remote copy both
support ipv6.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>