rust: qom: fix TODO about zeroability of classes

rust: qom: fix TODO about zeroability of classes

The proposed suggestion is not correct.  First it is not necessary for
*all* classes to be Zeroable, only for Rust-defined ones; classes
defined in C never implement ObjectImpl.

Second, the parent class field need not be Zeroable.  For example,
ChardevClass's chr_write and chr_be_event fields cannot be NULL,
therefore ChardevClass cannot be Zeroable.  However, char_class_init()
initializes them, therefore ChardevClass could be subclassed by Rust code.

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>