rust: qom: add reference counting functionality

Add a smart pointer that allows to add and remove references from
QOM objects.  It's important to note that while all QOM objects have a
reference count, in practice not all of them have their lifetime guarded
by it.  Embedded objects, specifically, are confined to the lifetime of
the owner.

When writing Rust bindings this is important, because embedded objects are
*never* used through the "Owned<>" smart pointer that is introduced here.

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

rust/qemu-api/src/vmstate.rs

@@ -29,6 +29,8 @@ use core::{marker::PhantomData, mem, ptr::NonNull};
 pub use crate::bindings::{VMStateDescription, VMStateField};
 use crate::{
     bindings::{self, VMStateFlags},
+    prelude::*,
+    qom::Owned,
     zeroable::Zeroable,
 };
 
@@ -191,7 +193,8 @@ pub const fn vmstate_varray_flag<T: VMState>(_: PhantomData<T>) -> VMStateFlags
 /// * a transparent wrapper for any of the above (`Cell`, `UnsafeCell`,
 ///   [`BqlCell`](crate::cell::BqlCell), [`BqlRefCell`](crate::cell::BqlRefCell)
 /// * a raw pointer to any of the above
-/// * a `NonNull` pointer or a `Box` for any of the above
+/// * a `NonNull` pointer, a `Box` or an [`Owned`](crate::qom::Owned) for any of
+///   the above
 /// * an array of any of the above
 ///
 /// In order to support other types, the trait `VMState` must be implemented
@@ -398,6 +401,7 @@ impl_vmstate_pointer!(NonNull<T> where T: VMState);
 // Unlike C pointers, Box is always non-null therefore there is no need
 // to specify VMS_ALLOC.
 impl_vmstate_pointer!(Box<T> where T: VMState);
+impl_vmstate_pointer!(Owned<T> where T: VMState + ObjectType);
 
 // Arrays using the underlying type's VMState plus
 // VMS_ARRAY/VMS_ARRAY_OF_POINTER