define the default storage pool if not defined

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