Virtual Disk images are often used on Windows and/or Mac platforms where
xz is not a widely known file ending and also not supported by dafault.
Use zip which is much better known.
Keep using xz for boards since those are not meant to be extracted by
users but directly used in Etcher. Also keep using xz for qcow2, since
qcow2 is mostly used on Linux platforms where xz is available by default
and zip usually needs an extra package.
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