balloon: Disassociate handlers from balloon device on unplug

When a balloon device gets unplugged, allow the balloon handlers to be
freed.

Reported-by: Shaolong Hu <shu@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

balloon.c

@@ -52,6 +52,16 @@ int qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
     return 0;
 }
 
+void qemu_remove_balloon_handler(void *opaque)
+{
+    if (balloon_opaque != opaque) {
+        return;
+    }
+    balloon_event_fn = NULL;
+    balloon_stat_fn = NULL;
+    balloon_opaque = NULL;
+}
+
 static int qemu_balloon(ram_addr_t target)
 {
     if (!balloon_event_fn) {

balloon.h

@@ -22,6 +22,7 @@ typedef void (QEMUBalloonStatus)(void *opaque, MonitorCompletion cb,
 
 int qemu_add_balloon_handler(QEMUBalloonEvent *event_func,
 			     QEMUBalloonStatus *stat_func, void *opaque);
+void qemu_remove_balloon_handler(void *opaque);
 
 void monitor_print_balloon(Monitor *mon, const QObject *data);
 int do_info_balloon(Monitor *mon, MonitorCompletion cb, void *opaque);