rust/qdev: Make REALIZE safe

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>