rust: qom: add ObjectImpl::CLASS_INIT

rust: qom: add ObjectImpl::CLASS_INIT

As shown in the PL011 device, the orphan rules required a manual
implementation of ClassInitImpl for anything not in the qemu_api crate;
this gets in the way of moving system emulation-specific code (including
DeviceClass, which as a blanket ClassInitImpl<DeviceClass> implementation)
into its own crate.

Make ClassInitImpl optional, at the cost of having to specify the CLASS_INIT
member by hand in every implementation of ObjectImpl.  The next commits will
get rid of it, replacing all the "impl<T> ClassInitImpl<Class> for T" blocks
with a generic class_init<T> method on Class.

Right now the definition is always the same, but do not provide a default
as that will not be true once ClassInitImpl goes away.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>