The use of 'lxc://' was mistakenly broken in:

  commit 4c8574c8
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Wed Mar 28 12:49:29 2018 +0100

    driver: ensure NULL URI isn't passed to drivers with whitelisted URIs

Allow it again for historical compatibility.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>