rust: qemu_api_macros: add Wrapper derive macro

Add a derive macro that makes it easy to peel off all the layers of
specialness (UnsafeCell, MaybeUninit, etc.) and just get a pointer
to the wrapped type; and likewise add them back starting from a
*mut.

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

docs/devel/rust.rst

@@ -315,11 +315,13 @@ a raw pointer, for use in calls to C functions.  It can be used for
 example as follows::
 
     #[repr(transparent)]
-    #[derive(Debug)]
+    #[derive(Debug, qemu_api_macros::Wrapper)]
     pub struct Object(Opaque<bindings::Object>);
 
-The bindings will then manually check for the big QEMU lock with
-assertions, which allows the wrapper to be declared thread-safe::
+where the special ``derive`` macro provides useful methods such as
+``from_raw``, ``as_ptr`, ``as_mut_ptr`` and ``raw_get``.  The bindings will
+then manually check for the big QEMU lock with assertions, which allows
+the wrapper to be declared thread-safe::
 
     unsafe impl Send for Object {}
     unsafe impl Sync for Object {}