qdev: Document that pointer properties kill device_add

Ask users of DEFINE_PROP_PTR() to set
cannot_instantiate_with_device_add_yet, or explain why it's not
needed.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

include/hw/qdev-properties.h

@@ -122,8 +122,25 @@ extern PropertyInfo qdev_prop_arraylen;
 #define DEFINE_PROP_PCI_DEVFN(_n, _s, _f, _d)                   \
     DEFINE_PROP_DEFAULT(_n, _s, _f, _d, qdev_prop_pci_devfn, int32_t)
 
+/*
+ * Please avoid pointer properties.  If you must use them, you must
+ * cover them in their device's class init function as follows:
+ *
+ * - If the property must be set, the device cannot be used with
+ *   device_add, so add code like this:
+ *   |* Reason: pointer property "NAME-OF-YOUR-PROP" *|
+ *   DeviceClass *dc = DEVICE_CLASS(class);
+ *   dc->cannot_instantiate_with_device_add_yet = true;
+ *
+ * - If the property may safely remain null, document it like this:
+ *   |*
+ *    * Note: pointer property "interrupt_vector" may remain null, thus
+ *    * no need for dc->cannot_instantiate_with_device_add_yet = true;
+ *    *|
+ */
 #define DEFINE_PROP_PTR(_n, _s, _f)             \
     DEFINE_PROP(_n, _s, _f, qdev_prop_ptr, void*)
+
 #define DEFINE_PROP_CHR(_n, _s, _f)             \
     DEFINE_PROP(_n, _s, _f, qdev_prop_chr, CharDriverState*)
 #define DEFINE_PROP_STRING(_n, _s, _f)             \