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/device_class.rs

@@ -6,6 +6,7 @@ use std::{ffi::CStr, os::raw::c_void};
 
 use crate::{
     bindings::{self, DeviceClass, DeviceState, Error, ObjectClass, Property, VMStateDescription},
+    prelude::*,
     zeroable::Zeroable,
 };
 
@@ -146,8 +147,8 @@ macro_rules! declare_properties {
     };
 }
 
-// workaround until we can use --generate-cstr in bindgen.
-pub const TYPE_DEVICE: &CStr =
-    unsafe { CStr::from_bytes_with_nul_unchecked(bindings::TYPE_DEVICE) };
-pub const TYPE_SYS_BUS_DEVICE: &CStr =
-    unsafe { CStr::from_bytes_with_nul_unchecked(bindings::TYPE_SYS_BUS_DEVICE) };
+unsafe impl ObjectType for bindings::DeviceState {
+    type Class = bindings::DeviceClass;
+    const TYPE_NAME: &'static CStr =
+        unsafe { CStr::from_bytes_with_nul_unchecked(bindings::TYPE_DEVICE) };
+}