netinfo: implement functions gathering IPv6 information

Change-Id: I2318ffa7d71abe3f57cb5e480bc46565e29f3894
Signed-off-by: huntxu <mhuntxu@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/9381
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>