Alas, I am now faced with a more revealing error:
http://dpaste.com/hold/119347/
Looking through storage_conf.c and storage_backend_logical.c, I believe
that my XML matches the XPath used on line 1005 of storage_conf.c:
char *format = virXPathString(conn,
"string(/volume/backingStore/format/@type)", ctxt);
I'll be posting a patch shortly which should fix this issue.
Thanks,
Cole