qom: Correct error values in two contracts

qom: Correct error values in two contracts

object_property_get_bool()'s contract claims it returns NULL on error.
Pasto; it returns false.

object_property_get_int()'s contract claims it returns "negative".  It
actually returns -1.  All the other object_property_get_FOO()
contracts specify the exact error value, so do the same here.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Greg Kurz <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Message-Id: <20200917125540[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>