spapr: initialize local Error pointer

This fixes a crash in the target QEMU during migration.

Broken in commit c5f54f3.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[reworded commit message]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
