Currently we hard code the vpc options `subformat=fixed` and
`force_size`, which are needed to generate valid azure images
with newer versions of qemu. But for other use cases or other
versions of qemu these options might not be wanted or valid.
Expose all the options but with defaults corresponding to the
old behavior.
Add a unit test for the `force_size` option to check its
effect. Also add a check for the correct size to the existing,
default value (i.e. `force_size` being `true`).
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