[libvirt] storage_backend_fs.c Bug

[Russell]

Line 1133 of storage_backend_fs.c in 0.6.0:

        if (vol->target.backingStore != NULL) {
            virStorageReportError(conn, VIR_ERR_NO_SUPPORT,
                                  _("copy-on-write image not supported with "
                                    "qcow-create"));
            return -1;
        }

virStorageVolTarget has no member called backingStore, resulting in
compile errors.