tests: qdev_prop_check_globals() doesn't return "all_used"

tests: qdev_prop_check_globals() doesn't return "all_used"

Instead, it returns 1 if an error was detected, which is the case for:

/qdev/properties/dynamic/global/subprocess:
warning: global dynamic-prop-type-bad.prop3 has invalid class name
warning: global nohotplug-type.prop5=105 not used
warning: global nondevice-type.prop6 has invalid class name

Clarify the function return value.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20181204142023.15982-2-marcandre.lureau@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>