The previous version of the code would only kick in if the state
directory path pointed at a *file*, which never occurs. Making that
codepath actually work reveals an ordering bug, which this patch fixes
as well.
It also replaces the confusing, imperative case log message "delete VM
state directory" with "deleting VM state directory".
Finally, we hint the user about how to prevent this deletion. IE. by
passing the --keep-vm-state flag.
Bug report:
https://github.com/NixOS/nixpkgs/pull/91046#issuecomment-685568750
Credit goes to Edef for the rebase on top of a recent nixpkgs commit
and for writing most of this commit message.
Co-authored-by: edef <edef@edef.eu>
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