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Looks like xend insists on this parameter being passed in the dump http message. Hacking it into xend_internal.c makes it work. This is with xen 4.0.1-2; utterly trivial to reproduce since all 'virsh dump' commands will fail.
Sorry this never received a response. Not sure if this was ever fixed, but since xend is deprecated now I don't know if this is still affecting anyone. If anyone can still reproduce with a recent libvirt setup, please reopen this