rust: qom: split ObjectType from ObjectImpl trait

Define a separate trait for fields that also applies to classes that are
defined by C code.  This makes it possible to add metadata to core classes,
which has multiple uses:

- it makes it possible to access the parent struct's TYPE_* for types
  that are defined in Rust code, and to avoid repeating it in every subclass

- implementors of ObjectType will be allowed to implement the IsA<> trait and
  therefore to perform typesafe casts from one class to another.

- in the future, an ObjectType could be created with Foo::new() in a type-safe
  manner, without having to pass a TYPE_* constant.

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

rust/qemu-api/src/sysbus.rs

@@ -2,11 +2,17 @@
 // Author(s): Paolo Bonzini <pbonzini@redhat.com>
 // SPDX-License-Identifier: GPL-2.0-or-later
 
-use std::ptr::addr_of;
+use std::{ffi::CStr, ptr::addr_of};
 
 pub use bindings::{SysBusDevice, SysBusDeviceClass};
 
-use crate::{bindings, cell::bql_locked, irq::InterruptSource};
+use crate::{bindings, cell::bql_locked, irq::InterruptSource, prelude::*};
+
+unsafe impl ObjectType for SysBusDevice {
+    type Class = SysBusDeviceClass;
+    const TYPE_NAME: &'static CStr =
+        unsafe { CStr::from_bytes_with_nul_unchecked(bindings::TYPE_SYS_BUS_DEVICE) };
+}
 
 impl SysBusDevice {
     /// Return `self` cast to a mutable pointer, for use in calls to C code.