The linter is complaining the `pick_default_qemu_bin` is not explicitly
returning None. Fix it to explicitly return None and avoid R1710
inconsistent-return-statements.

Signed-off-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210920204932.94132-6-willianr@redhat.com>