Change behaviour of OpDiagnoseOS w.r.t. 'valid'

This patch changes the behaviour of OpDiagnoseOS with regards to the
'valid' field to be similar to the one for the hidden/blacklisted
fields: unless this field is requested, invalid OSes are filtered out.

The rationale is that, except for the gnt-os info/diagnose, all other
users of this opcode are requesting the valid field just to filter out
invalid OSes, and not for any other use. Thus, changing this behaviour
makes these callers simpler.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>