docs/devel: mention the spacing requirement for QOM

We have a more complete document on QOM but we should at least mention
the style requirements in the style guide.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20230424092249.58552-18-alex.bennee@linaro.org>

docs/devel/qom.rst

+.. _qom:
+
 ===========================
 The QEMU Object Model (QOM)
 ===========================

docs/devel/style.rst

@@ -628,6 +628,43 @@ are still some caveats to beware of
 QEMU Specific Idioms
 ********************
 
+QEMU Object Model Declarations
+==============================
+
+The QEMU Object Model (QOM) provides a framework for handling objects
+in the base C language. The first declaration of a storage or class
+structure should always be the parent and leave a visual space between
+that declaration and the new code. It is also useful to separate
+backing for properties (options driven by the user) and internal state
+to make navigation easier.
+
+For a storage structure the first declaration should always be called
+"parent_obj" and for a class structure the first member should always
+be called "parent_class" as below:
+
+.. code-block:: c
+
+    struct MyDeviceState {
+        DeviceState parent_obj;
+
+        /* Properties */
+        int prop_a;
+        char *prop_b;
+        /* Other stuff */
+        int internal_state;
+    };
+
+    struct MyDeviceClass {
+        DeviceClass parent_class;
+
+        void (*new_fn1)(void);
+        bool (*new_fn2)(CPUState *);
+    };
+
+Note that there is no need to provide typedefs for QOM structures
+since these are generated automatically by the QOM declaration macros.
+See :ref:`qom` for more details.
+
 Error handling and reporting
 ============================