define the default storage pool if its not there yet

On some OS's (e.g. Ubuntu 13.04) there isn't a default libvirt storage pool defined out of the box (e.g. https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1018956) Virt-manager appears to define one automatically (https://www.redhat.com/archives/virt-tools-list/2010-August/msg00012.html) but if you are not using that you need to make sure the pool is around before calling vol-create-as