[mdns] support V6 addresses on non-V4-provisioned NICs

This CL fixes a bug in mdns that prevented it from using the V6
aspect of a NIC if the NIC has not been provisioned with a V4
address.

TEST: watch logs during boot sequence, see V6 address in use
      before V4 provisioning:
[32.931] 07924.07940> Starting mDNS on interface ethx9a [80fe::ae8e:ff4c:eefe:9a40]
[37.934] 06251.14608> netstack: netstack.go:214: NIC ethx9a: DHCP acquired IP 192.168.42.188 for 24h0m0s
[37.934] 07924.07940> Starting mDNS on interface ethx9a 192.168.42.188

Change-Id: I3049a7ae4a9a1fe9e0dfb0dce30ae5227a2ca03a
1 file changed