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/tests/tests.rs

@@ -5,12 +5,8 @@
 use std::ffi::CStr;
 
 use qemu_api::{
-    bindings::*,
-    c_str, declare_properties, define_property,
-    definitions::ObjectImpl,
-    device_class::{self, DeviceImpl},
-    impl_device_class,
-    zeroable::Zeroable,
+    bindings::*, c_str, declare_properties, define_property, definitions::ObjectImpl,
+    device_class::DeviceImpl, impl_device_class, prelude::*, zeroable::Zeroable,
 };
 
 #[test]
@@ -46,10 +42,15 @@ fn test_device_decl_macros() {
             ),
     }
 
-    impl ObjectImpl for DummyState {
+    unsafe impl ObjectType for DummyState {
         type Class = DummyClass;
         const TYPE_NAME: &'static CStr = c_str!("dummy");
-        const PARENT_TYPE_NAME: Option<&'static CStr> = Some(device_class::TYPE_DEVICE);
+    }
+
+    impl ObjectImpl for DummyState {
+        const PARENT_TYPE_NAME: Option<&'static CStr> =
+            Some(<DeviceState as ObjectType>::TYPE_NAME);
+        const ABSTRACT: bool = false;
     }
 
     impl DeviceImpl for DummyState {