2.2.1 Legacy Interface:

2.2.1 Legacy Interface... reads in part:

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Legacy Interface
is an interface specified by an earlier draft of this specification (up to 0.9.X)
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While what appears to be earlier work is cited on the cover page, there is no entry in the normative references for: http://ozlabs.org/~rusty/virtio-spec/virtio-0.9.5.pdf or any other prior versions.

What that means is that an implementer of your specification is not required to conform to any prior version because it isn't in the normative references nor is it cited in the text. Both of those are necessary for an implementer to be required to conform to another specification. Normative reference + specific citation in the text.

Assume that you insert the normative references, then you need to reference the parts of that reference, that establish what someone must do in accordance with the other specification. I am assuming it is the prior definition of an interface. So cite that section or sections necessary for that.

Otherwise, you have no way to reference the other, legacy specification.