qmp: Give saner messages related to qmp_capabilities misuse

Pretending that QMP doesn't understand a command merely because
we are not in the right mode doesn't help first-time users figure
out what to do to correct things.  Although the documentation for
QMP calls out capabilities negotiation, we should also make it
clear in our error messages what we were expecting.  With this
patch, I now get the following transcript:

$ ./x86_64-softmmu/qemu-system-x86_64 -qmp stdio -nodefaults
{"QMP": {"version": {"qemu": {"micro": 93, "minor": 2, "major": 2}, "package": ""}, "capabilities": []}}
{"execute":"huh"}
{"error": {"class": "CommandNotFound", "desc": "The command huh has not been found"}}
{"execute":"quit"}
{"error": {"class": "CommandNotFound", "desc": "Expecting capabilities negotiation with 'qmp_capabilities' before command 'quit'"}}
{"execute":"qmp_capabilities"}
{"return": {}}
{"execute":"qmp_capabilities"}
{"error": {"class": "CommandNotFound", "desc": "Capabilities negotiation is already complete, command 'qmp_capabilities' ignored"}}
{"execute":"quit"}
{"return": {}}
{"timestamp": {"seconds": 1429110729, "microseconds": 181935}, "event": "SHUTDOWN"}

Signed-off-by: Eric Blake <eblake@redhat.com>
Tested-By: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Paulo Vital <paulo.vital@profitbricks.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -4783,10 +4783,22 @@ static int monitor_can_read(void *opaque)
     return (mon->suspend_cnt == 0) ? 1 : 0;
 }
 
-static int invalid_qmp_mode(const Monitor *mon, const mon_cmd_t *cmd)
+static bool invalid_qmp_mode(const Monitor *mon, const mon_cmd_t *cmd)
 {
-    int is_cap = cmd->mhandler.cmd_new == do_qmp_capabilities;
-    return (qmp_cmd_mode(mon) ? is_cap : !is_cap);
+    bool is_cap = cmd->mhandler.cmd_new == do_qmp_capabilities;
+    if (is_cap && qmp_cmd_mode(mon)) {
+        qerror_report(ERROR_CLASS_COMMAND_NOT_FOUND,
+                      "Capabilities negotiation is already complete, command "
+                      "'%s' ignored", cmd->name);
+        return true;
+    }
+    if (!is_cap && !qmp_cmd_mode(mon)) {
+        qerror_report(ERROR_CLASS_COMMAND_NOT_FOUND,
+                      "Expecting capabilities negotiation with "
+                      "'qmp_capabilities' before command '%s'", cmd->name);
+        return true;
+    }
+    return false;
 }
 
 /*
@@ -5080,11 +5092,14 @@ static void handle_qmp_command(JSONMessageParser *parser, QList *tokens)
     cmd_name = qdict_get_str(input, "execute");
     trace_handle_qmp_command(mon, cmd_name);
     cmd = qmp_find_cmd(cmd_name);
-    if (!cmd || invalid_qmp_mode(mon, cmd)) {
+    if (!cmd) {
         qerror_report(ERROR_CLASS_COMMAND_NOT_FOUND,
                       "The command %s has not been found", cmd_name);
         goto err_out;
     }
+    if (invalid_qmp_mode(mon, cmd)) {
+        goto err_out;
+    }
 
     obj = qdict_get(input, "arguments");
     if (!obj) {