vdo: fix input units for minimim_io_size

vdo: fix input units for minimim_io_size

When specifying minimum_io_size with --vdosettings,
command assumed wrong unit (sectors).

So '--vdosettings minimum_io_size=512|4096' resulted into
an error that only 512 or 4096 values are allowed, but
at the same time values  1 or 8 were accepted.

So fix by converting any number >= 512 to 'sectors' and
keep input of 1 or 8 still valid if anyone has been using
this before.

So now we take  512 or 4096  and still also  1 or 8 with the
same effect.

Also correct the 'error' message when invalid minimum_io_size
is specified.