nx: make pack string of IPv6 address consistent with OF1.2/1.3

Changed pack string of IPv6 address in NXM from former '!4I' to
'!8H'. Registered IPv6 src/dst field header so we could parse IPv6
addresses now.

Signed-off-by: Can Zhang <can@canx.me>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>