virt-sysprep problem

Question

Virt-sysprep man page notes :

You do not need to run virt-sysprep as root. In fact we'd generally recommend that you don't. The time you might want to run it as root is when you need root in order to access the disk image, but even in this case it would be better to change the permissions on the disk image to be writable as the non-root user running virt-sysprep.

But when I run virt-sysprep from my user account, I get the following :

$ virt-sysprep -a asdf.vdi
Examining the guest ...
libguestfs: warning: supermin-helper -f checksum returned a short string Fatal error: exception Guestfs.Error("cannot find any suitable libguestfs supermin, fixed or old-style appliance on LIBGUESTFS_PATH (search path: /usr/lib/guestfs)")

Whereas running with root privileges seems to work :

$ sudo virt-sysprep -a asdf.vdi
[sudo] password for user:
Examining the guest
Performing "yum-uuid" Performing "utmp" Performing "udev-persistent-net"
Performing "tmp-files"
Performing "sssd-db-log"
Performing "ssh-userdir"
Performing "ssh-hostkeys" Performing "smolt-uuid"
Performing "script"
Performing "samba-db-log"
Performing "rpm-db"
Performing "rhn-systemid" Performing "random-seed"
Performing "puppet-data-log"
Performing "password"
Performing "pam-data"
Performing "package-manager-cache" Performing "pacct-log"
Performing "net-hwaddr"
Performing "net-hostname"
Performing "mail-spool"
Performing "machine-id" Performing "logfiles"
Performing "hostname"
Performing "firstboot" Performing "dovecot-data"
Performing "dhcp-server-state"
Performing "dhcp-client-state"
Performing "cron-spool"
Performing "crash-data" Performing "blkid-tab"
Performing "bash-history"
Performing "abrt-data" Performing "lvm-uuids"

What gives ?
(Ubuntu 14.04.2 64-bit)