The use of 'lxc://' was mistakenly broken in:
commit 4c8574c85c554e68de0d8bf9b85727954a5bea91
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: Daniel P. Berrangé <berrange@redhat.com>