monitor: Avoid readline functions in QMP

The monitor_read_command() function is readline specific
and should only be used when readline is available.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

monitor.c

@@ -140,6 +140,9 @@ static inline int monitor_ctrl_mode(const Monitor *mon)
 
 static void monitor_read_command(Monitor *mon, int show_prompt)
 {
+    if (!mon->rs)
+        return;
+
     readline_start(mon->rs, "(qemu) ", 0, monitor_command_cb, NULL);
     if (show_prompt)
         readline_show_prompt(mon->rs);