rust: qom: add casting functionality

Add traits that let client cast typecast safely between object types.
In particular, an upcast is compile-time guaranteed to succeed, and a
YOLO C-style downcast must be marked as unsafe.

The traits are based on an IsA<> trait that declares what
is a subclass of what, which is an idea taken from glib-rs
(https://docs.rs/glib/latest/glib/object/trait.IsA.html).
The four primitives are also taken from there
(https://docs.rs/glib/latest/glib/object/trait.Cast.html).  However,
the implementation of casting itself is a bit different and uses the
Deref trait.

This removes some pointer arithmetic from the pl011 device; it is also a
prerequisite for the definition of methods, so that they can be invoked
on all subclass structs.  This will use the IsA<> trait to detect the
structs that support the methods.

glib also has a "monadic" casting trait which could be implemented on
Option (as in https://docs.rs/glib/latest/glib/object/trait.CastNone.html)
and perhaps even Result.  For now I'm leaving it out, as the patch is
already big enough and the benefit seems debatable.

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

rust/qemu-api/src/sysbus.rs

@@ -7,7 +7,11 @@ use std::{ffi::CStr, ptr::addr_of};
 pub use bindings::{SysBusDevice, SysBusDeviceClass};
 
 use crate::{
-    bindings, cell::bql_locked, irq::InterruptSource, prelude::*, qdev::DeviceClass,
+    bindings,
+    cell::bql_locked,
+    irq::InterruptSource,
+    prelude::*,
+    qdev::{DeviceClass, DeviceState},
     qom::ClassInitImpl,
 };
 
@@ -16,6 +20,7 @@ unsafe impl ObjectType for SysBusDevice {
     const TYPE_NAME: &'static CStr =
         unsafe { CStr::from_bytes_with_nul_unchecked(bindings::TYPE_SYS_BUS_DEVICE) };
 }
+qom_isa!(SysBusDevice: DeviceState, Object);
 
 // TODO: add SysBusDeviceImpl
 impl<T> ClassInitImpl<SysBusDeviceClass> for T