can't use adt-build-lxc in xenial

[Leo Arias]

I'm trying to create a lxc machine for adt-run following the insctructions from the manual, but it fails:

⟫ adt-build-lxc ubuntu xenial
best is not a valid backing storage type.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: autopkgtest 3.20.4
ProcVersionSignature: Ubuntu 4.4.0-15.31-generic 4.4.6
Uname: Linux 4.4.0-15-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
Date: Thu May 5 18:32:21 2016
Ec2AMI: ami-0000103f
Ec2AMIManifest: FIXME
Ec2AvailabilityZone: nova
Ec2InstanceType: cpu10-ram10-disk100-ephemeral20
Ec2Kernel: aki-0000022a
Ec2Ramdisk: ari-0000022a
JournalErrors:
 Error: command ['journalctl', '-b', '--priority=warning', '--lines=1000'] failed with exit code 1: Hint: You are currently not seeing messages from other users and the system.
       Users in the 'systemd-journal' group can see all messages. Pass -q to
       turn off this notice.
 No journal files were opened due to insufficient permissions.
PackageArchitecture: all
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: autopkgtest
UpgradeStatus: No upgrade log present (probably fresh install)