kern: ether_gen_addr: randomize on default hostuuid, too

Currently, this will still hash the default (all zero) hostuuid and
potentially arrive at a MAC address that has a high chance of collision
if another interface of the same name appears in the same broadcast
domain on another host without a hostuuid, e.g., some virtual machine
setups.

Instead of using the default hostuuid, just treat it as a failure and
generate a random LA unicast MAC address.

Reviewed by: bz, gbe, imp, kbowling, kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29788

(cherry picked from commit 2d741f33bd07bf94a59635db3c7b9e070a8a6e55)