Max: Did you have in mind something like this?

v2:
 - Fix qemu-img from now choking when it gets a partial response.

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For convenience, this branch is available at:
https://github.com/jnsnow/qemu.git branch block-allow-partial-query
https://github.com/jnsnow/qemu/tree/block-allow-partial-query

This version is tagged block-allow-partial-query-v2:
https://github.com/jnsnow/qemu/releases/tag/block-allow-partial-query-v2