docs: create interop/ subdirectory

This is for the future interoperability & management guide.  It includes
the QAPI docs, including the automatically generated ones, other socket
protocols (vhost-user, VNC), and the qcow2 file format.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/interop/qmp-intro.txt

@@ -0,0 +1,86 @@
+                          QEMU Machine Protocol
+                          =====================
+
+Introduction
+------------
+
+The QEMU Machine Protocol (QMP) allows applications to operate a
+QEMU instance.
+
+QMP is JSON[1] based and features the following:
+
+- Lightweight, text-based, easy to parse data format
+- Asynchronous messages support (ie. events)
+- Capabilities Negotiation
+
+For detailed information on QMP's usage, please, refer to the following files:
+
+o qmp-spec.txt      QEMU Machine Protocol current specification
+o qemu-qmp-ref.html QEMU QMP commands and events (auto-generated at build-time)
+
+[1] http://www.json.org
+
+Usage
+-----
+
+You can use the -qmp option to enable QMP. For example, the following
+makes QMP available on localhost port 4444:
+
+$ qemu [...] -qmp tcp:localhost:4444,server,nowait
+
+However, for more flexibility and to make use of more options, the -mon
+command-line option should be used. For instance, the following example
+creates one HMP instance (human monitor) on stdio and one QMP instance
+on localhost port 4444:
+
+$ qemu [...] -chardev stdio,id=mon0 -mon chardev=mon0,mode=readline \
+             -chardev socket,id=mon1,host=localhost,port=4444,server,nowait \
+             -mon chardev=mon1,mode=control,pretty=on
+
+Please, refer to QEMU's manpage for more information.
+
+Simple Testing
+--------------
+
+To manually test QMP one can connect with telnet and issue commands by hand:
+
+$ telnet localhost 4444
+Trying 127.0.0.1...
+Connected to localhost.
+Escape character is '^]'.
+{
+    "QMP": {
+        "version": {
+            "qemu": {
+                "micro": 50, 
+                "minor": 6, 
+                "major": 1
+            }, 
+            "package": ""
+        }, 
+        "capabilities": [
+        ]
+    }
+}
+
+{ "execute": "qmp_capabilities" }
+{
+    "return": {
+    }
+}
+
+{ "execute": "query-status" }
+{
+    "return": {
+        "status": "prelaunch", 
+        "singlestep": false, 
+        "running": false
+    }
+}
+
+Please, refer to the qapi-schema.json file for a complete command reference.
+
+QMP wiki page
+-------------
+
+http://wiki.qemu-project.org/QMP