docs: Deprecate incorrectly typed device_add arguments

While introducing a non-QemuOpts code path for device creation for JSON
-device, we noticed that QMP device_add doesn't check its input
correctly (accepting arguments that should have been rejected), and that
users may be relying on this behaviour (libvirt did until it was fixed
recently).

Let's use a deprecation period before we fix this bug in QEMU to avoid
nasty surprises for users.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211111143530.18985-1-kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211115145409.176785-12-kwolf@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

docs/about/deprecated.rst

@@ -250,6 +250,20 @@ options are removed in favor of using explicit ``blockdev-create`` and
 ``blockdev-add`` calls. See :doc:`/interop/live-block-operations` for
 details.
 
+Incorrectly typed ``device_add`` arguments (since 6.2)
+''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+Due to shortcomings in the internal implementation of ``device_add``, QEMU
+incorrectly accepts certain invalid arguments: Any object or list arguments are
+silently ignored. Other argument types are not checked, but an implicit
+conversion happens, so that e.g. string values can be assigned to integer
+device properties or vice versa.
+
+This is a bug in QEMU that will be fixed in the future so that previously
+accepted incorrect commands will return an error. Users should make sure that
+all arguments passed to ``device_add`` are consistent with the documented
+property types.
+
 System accelerators
 -------------------