Fix the viridian parent locators bug workaround

The 'platform_data_space' is supposed to be marshalled in sectors,
but some non-compliant vhds marshal it in bytes. Since the data is
a filename and therefore fairly short, we assume that any length
< 512 *sectors* is actually in sectors (as expected) while any length
>= 512 *sectors* is more likely to be an old vhd using bytes.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>