You need to upgrade to the latest LTS version of Ubuntu (16.04).
I refer to my message to this mailing list on 17/03/2016:
Following this user mailing list over a period of time, I saw a few messages regarding not being able using the external snapshot feature.
I refer to this procedure: http://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit
I would like to mention this hoping it will help those users:
Using external snapshots works perfect again in qemu 2.5.0 together with this patch https://bugzilla.redhat.com/show_bug.cgi?id=1300209
Full version list on which I did my testing:
Compiled against library: libvirt 1.3.1
Using library: libvirt 1.3.1
Gebruikte API: QEMU 1.3.1
Draaiende hypervisor: QEMU 2.5.0