Comment 23 for bug 1372368

With a standard trusty install, the versions installed are:
uvtool 0~bzr92-0ubuntu1
libvirt0 1.2.2-0ubuntu13.1.20

this combination does not have this problem, as libvirt hasn't yet changed to use type 'iso'.

After apt-add-repository cloud-archive:mitaka, and upgrading libvirt, the versions are:
uvtool 0~bzr92-0ubuntu1
libvirt0 1.3.1-1ubuntu10.8~cloud0

this combination does show this problem:

$ uvt-kvm create uca-test release=xenial arch=amd64
uvt-kvm: error: libvirt: internal error: process exited while connecting to monitor: 2017-04-17T16:21:41.224870Z qemu-system-x86_64: -drive file=/var/lib/uvtool/libvirt/images/uca-test-ds.qcow,format=iso,if=none,id=drive-virtio-disk1: 'iso' invalid format

upgrading uvtool (and uvtool-libvirt) does fix the problem, with versions:
uvtool 0~bzr92-0ubuntu1.1
uvtool-libvirt 0~bzr92-0ubuntu1.1
libvirt0 1.3.1-1ubuntu10.8~cloud0