Add 'query-events' command to QMP to query async events

Sometimes it is neccessary for an application to determine
whether a particular QMP event is available, so they can
decide whether to use compatibility code instead. This
introduces a new 'query-events' command to QMP to do just
that

 { "execute": "query-events" }
 {"return": [{"name": "WAKEUP"},
             {"name": "SUSPEND"},
             {"name": "DEVICE_TRAY_MOVED"},
             {"name": "BLOCK_JOB_CANCELLED"},
             {"name": "BLOCK_JOB_COMPLETED"},
             ...snip...
             {"name": "SHUTDOWN"}]}

* monitor.c: Turn MonitorEvent -> string conversion
  into a lookup from a static table of constant strings.
  Add impl of qmp_query_events monitor command handler
* qapi-schema.json, qmp-commands.hx: Define contract of
  query-events command

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

qmp-commands.hx

@@ -1208,6 +1208,43 @@ EQMP
         .mhandler.cmd_new = qmp_marshal_input_query_commands,
     },
 
+SQMP
+query-events
+--------------
+
+List QMP available events.
+
+Each event is represented by a json-object, the returned value is a json-array
+of all events.
+
+Each json-object contains:
+
+- "name": event's name (json-string)
+
+Example:
+
+-> { "execute": "query-events" }
+<- {
+      "return":[
+         {
+            "name":"SHUTDOWN"
+         },
+         {
+            "name":"RESET"
+         }
+      ]
+   }
+
+Note: This example has been shortened as the real response is too long.
+
+EQMP
+
+    {
+        .name       = "query-events",
+        .args_type  = "",
+        .mhandler.cmd_new = qmp_marshal_input_query_events,
+    },
+
 SQMP
 query-chardev
 -------------