qmp: add and use q type specifier

"O" is being used by the transaction and qom-set commands to mean "any
QObject", but it really means "do not validate the argument list".
Add a new specifier with the correct meaning.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
