nicira-ext: Support matching IPv6 traffic.

nicira-ext: Support matching IPv6 traffic.

Provides ability to match over IPv6 traffic in the same manner as IPv4.
Currently, the matching fields include:

    - IPv6 source and destination addresses (ipv6_src and ipv6_dst)
    - Traffic Class (nw_tos)
    - Next Header (nw_proto)
    - ICMPv6 Type and Code (icmp_type and icmp_code)
    - TCP and UDP Ports over IPv6 (tp_src and tp_dst)

When defining IPv6 rules, the Nicira Extensible Match (NXM) extension to
OVS must be used.

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>