[Suggestion from Eric Blake via libvir-list]


When creating an empty qcow2 overlay with 'virsh vol-create-as', as of now, disk image size needs to be specified, e.g.:

  $ virsh vol-create-as default base.qcow2 1G \
      --format qcow2 \
      --backing-vol overlay.qcow2 --backing-vol-format qcow2


Libvirt should be taught to not *require* to a size.


[*] http://www.redhat.com/archives/libvir-list/2014-October/msg00239.html