monitor: Establish cmd flags and convert the async tag

As we want to add more flags to monitor commands, convert the only so
far existing one accordingly.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/monitor.h b/monitor.h
index ea15469..9582b9c 100644
--- a/monitor.h
+++ b/monitor.h
@@ -15,6 +15,9 @@
 #define MONITOR_USE_READLINE  0x02
 #define MONITOR_USE_CONTROL   0x04
 
+/* flags for monitor commands */
+#define MONITOR_CMD_ASYNC       0x0001
+
 /* QMP events */
 typedef enum MonitorEvent {
     QEVENT_SHUTDOWN,