block: Open by reference will try device then node_name. Since we introduced node_name for named bs of the graph modify the opening by reference to use it as a fallback. This patch also enforce the separation of the device id and graph node namespaces. Signed-off-by: Benoit Canet <benoit@irqsave.net> Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>