docs: move D-Bus VMState documentation to source XML

Use the source XML document as single reference, importing its
documentation via the dbus-doc directive.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>

backends/dbus-vmstate1.xml

-<?xml version="1.0"?>
+<?xml version="1.0" encoding="utf-8"?>
 <node name="/" xmlns:doc="http://www.freedesktop.org/dbus/1.0/doc.dtd">
+  <!--
+      org.qemu.VMState1:
+
+      This interface must be implemented at the object path
+      ``/org/qemu/VMState1`` to support helper migration.
+  -->
   <interface name="org.qemu.VMState1">
+
+    <!--
+        Id:
+
+        A string that identifies the helper uniquely. (maximum 256 bytes
+        including terminating NUL byte)
+
+        .. note::
+
+           The VMState helper ID namespace is its own namespace. In particular,
+           it is not related to QEMU "id" used in -object/-device objects.
+    -->
     <property name="Id" type="s" access="read"/>
+
+    <!--
+        Load:
+        @data: data to restore the state.
+
+        The method called on destination with the state to restore.
+
+        The helper may be initially started in a waiting state (with an
+        ``-incoming`` argument for example), and it may resume on success.
+
+        An error may be returned to the caller.
+    -->
     <method name="Load">
       <arg type="ay" name="data" direction="in"/>
     </method>
+
+    <!--
+        Save:
+        @data: state data to save for later resume.
+
+        The method called on the source to get the current state to be
+        migrated. The helper should continue to run normally.
+
+        An error may be returned to the caller.
+    -->
     <method name="Save">
       <arg type="ay" name="data" direction="out"/>
     </method>

docs/interop/dbus-vmstate.rst

@@ -2,9 +2,6 @@
 D-Bus VMState
 =============
 
-Introduction
-============
-
 The QEMU dbus-vmstate object's aim is to migrate helpers' data running
 on a QEMU D-Bus bus. (refer to the :doc:`dbus` document for
 some recommendations on D-Bus usage)
@@ -26,49 +23,16 @@ dbus-vmstate object can be configured with the expected list of
 helpers by setting its ``id-list`` property, with a comma-separated
 ``Id`` list.
 
-Interface
-=========
-
-On object path ``/org/qemu/VMState1``, the following
-``org.qemu.VMState1`` interface should be implemented:
-
-.. code:: xml
-
-  <interface name="org.qemu.VMState1">
-    <property name="Id" type="s" access="read"/>
-    <method name="Load">
-      <arg type="ay" name="data" direction="in"/>
-    </method>
-    <method name="Save">
-      <arg type="ay" name="data" direction="out"/>
-    </method>
-  </interface>
-
-"Id" property
--------------
-
-A string that identifies the helper uniquely. (maximum 256 bytes
-including terminating NUL byte)
-
-.. note::
-
-   The helper ID namespace is a separate namespace. In particular, it is not
-   related to QEMU "id" used in -object/-device objects.
-
-Load(in u8[] bytes) method
---------------------------
-
-The method called on destination with the state to restore.
+.. only:: sphinx4
 
-The helper may be initially started in a waiting state (with
-an --incoming argument for example), and it may resume on success.
+   .. dbus-doc:: backends/dbus-vmstate1.xml
 
-An error may be returned to the caller.
+.. only:: not sphinx4
 
-Save(out u8[] bytes) method
----------------------------
+   .. warning::
+      Sphinx 4 is required to build D-Bus documentation.
 
-The method called on the source to get the current state to be
-migrated. The helper should continue to run normally.
+      This is the content of ``backends/dbus-vmstate1.xml``:
 
-An error may be returned to the caller.
+   .. literalinclude:: ../../backends/dbus-vmstate1.xml
+      :language: xml