Comment 2 for bug 1650651

As of Juju 2.1, Juju will look for an local image with the alias "juju/$series/$arch", and use that if it exists; else it will fetch it from cloud-images and give it that alias.

So change your import command to:

    lxc image import xenial-server-cloudimg-amd64-lxd.tar.xz xenial-server-cloudimg-amd64-root.tar.xz --alias juju/xenial/amd64.

and you should be good.