This error was encountered while trying to start a Red Hat DomU on a JS21 in an IBM Bladecenter.
The cause wasn’t immediately evident so it is captured here should it be encountered again.
The problem was caused, in this case, by the xen config file. It still contained the cd-rom stanza where the single dvd-rom drive was attached to a different blade.
Removing this stanza allowed the DomU to successfully boot.