Comment 3 for bug 1459748

I don't think this is related to qemu, it may be a libvirt change.

A simpler reproducer:
echo "<volume><name>test</name><target><format type="qcow2"/></target><capacity>0</capacity></volume>" > vol.xml
virsh vol-create uvtool vol.xml

shows the error:
error: internal error: Child process (/usr/bin/qemu-img create -f qcow2 -o compat=0.10 /var/lib/uvtool/libvirt/images/test) unexpected exit status 1: qemu-img: /var/lib/uvtool/libvirt/images/test: Image creation needs a size parameter

However if you run the same command in an older version this works.
The qemu-img command fails in all versions, so I suspect a libvirt change.