QMP: Introduce the qmp_capabilities command

This command will be used to enable QMP capabilities advertised
by the capabilities array.

Note that it will be mandatory to issue this command in order
to make QMP functional (although this behavior is not being
enforced by this commit).

Also, as we don't have any capabilities yet, the new command
doesn't accept any arguments. I will postpone the decision for
a format for this until we get our first capability.

Finally, this command is visible from the user Monitor too, in
the meaning that you can execute it but it won't do anything.
Making it only visible in QMP is beyond this series' goal, as
it requires changes in unrelated places.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

monitor.c

@@ -122,6 +122,7 @@ typedef struct MonitorControl {
     QObject *id;
     int print_enabled;
     JSONMessageParser parser;
+    int command_mode;
 } MonitorControl;
 
 struct Monitor {
@@ -412,6 +413,15 @@ void monitor_protocol_event(MonitorEvent event, QObject *data)
     QDECREF(qmp);
 }
 
+static void do_qmp_capabilities(Monitor *mon, const QDict *params,
+                                QObject **ret_data)
+{
+    /* Will setup QMP capabilities in the future */
+    if (monitor_ctrl_mode(mon)) {
+        mon->mc->command_mode = 1;
+    }
+}
+
 static int compare_cmd(const char *name, const char *list)
 {
     const char *p, *pstart;
@@ -4385,6 +4395,7 @@ static void monitor_control_event(void *opaque, int event)
         QObject *data;
         Monitor *mon = opaque;
 
+        mon->mc->command_mode = 0;
         json_message_parser_init(&mon->mc->parser, handle_qmp_command);
 
         data = get_qmp_greeting();

qemu-monitor.hx

@@ -1124,6 +1124,20 @@ STEXI
 @item block_passwd @var{device} @var{password}
 @findex block_passwd
 Set the encrypted device @var{device} password to @var{password}
+ETEXI
+
+    {
+        .name       = "qmp_capabilities",
+        .args_type  = "",
+        .params     = "",
+        .help       = "enable QMP capabilities",
+        .user_print = monitor_user_noop,
+        .mhandler.cmd_new = do_qmp_capabilities,
+    },
+
+STEXI
+@item qmp_capabilities
+Enable the specified QMP capabilities
 ETEXI
 
 STEXI