virttest: Setup ipv6 env to make all cases works with ipv6

This patch setup ipv6 env in virttest, if you want using
ipv6 in your test, set "ip_version = IPV6" in your test cfg.

During test using ipv6 global address as default, if your test
env not support ipv6 glbal address auto config, and/or you want
using linklocal address, set "using_linklocal = yes".

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
virttest.test.ping: Make ping test support ipv6

This patch make linux ping test support ipv6.

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