rust/qdev: Make REALIZE safe

A safe REALIZE accepts immutable reference.

Since current PL011's realize() only calls a char binding function (
qemu_chr_fe_set_handlers), it is possible to convert mutable reference
(&mut self) to immutable reference (&self), which only needs to convert
the pointers passed to C to mutable pointers.

Thus, make REALIZE accept immutable reference.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250121140457.84631-2-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/char/pl011/src/device.rs

@@ -2,7 +2,7 @@
 // Author(s): Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
 // SPDX-License-Identifier: GPL-2.0-or-later
 
-use core::ptr::{addr_of_mut, NonNull};
+use core::ptr::{addr_of, addr_of_mut, NonNull};
 use std::{
     ffi::CStr,
     os::raw::{c_int, c_uint, c_void},
@@ -156,7 +156,7 @@ fn properties() -> &'static [Property] {
     fn vmsd() -> Option<&'static VMStateDescription> {
         Some(&device_class::VMSTATE_PL011)
     }
-    const REALIZE: Option<fn(&mut Self)> = Some(Self::realize);
+    const REALIZE: Option<fn(&Self)> = Some(Self::realize);
     const RESET: Option<fn(&mut Self)> = Some(Self::reset);
 }
 
@@ -439,17 +439,17 @@ fn set_read_trigger(&mut self) {
         self.read_trigger = 1;
     }
 
-    pub fn realize(&mut self) {
+    pub fn realize(&self) {
         // SAFETY: self.char_backend has the correct size and alignment for a
         // CharBackend object, and its callbacks are of the correct types.
         unsafe {
             qemu_chr_fe_set_handlers(
-                addr_of_mut!(self.char_backend),
+                addr_of!(self.char_backend) as *mut CharBackend,
                 Some(pl011_can_receive),
                 Some(pl011_receive),
                 Some(pl011_event),
                 None,
-                addr_of_mut!(*self).cast::<c_void>(),
+                addr_of!(*self).cast::<c_void>() as *mut c_void,
                 core::ptr::null_mut(),
                 true,
             );

rust/qemu-api/src/qdev.rs

@@ -23,7 +23,7 @@ pub trait DeviceImpl {
     ///
     /// If not `None`, the parent class's `realize` method is overridden
     /// with the function pointed to by `REALIZE`.
-    const REALIZE: Option<fn(&mut Self)> = None;
+    const REALIZE: Option<fn(&Self)> = None;
 
     /// If not `None`, the parent class's `reset` method is overridden
     /// with the function pointed to by `RESET`.