The user must be aware of any mistake he did. For example, he may lose
time by not understanding why the VM does not behave as expected or by
distorting measures in benchmarks.

The only legitimate reason to automatically fix a parameter is backward
compatibility, if a value is not valid anymore. But such fallback
should only be temporary. There's no such case recently.

At last, it increased maintenance by having to keep the lists of valid
values up-to-date.

About:

  -  if disk_info['io'] == 'native':
  -    additional_disk_options += ',cache.direct=on'

These lines are redundant when cache is none.