qmp: Check for returned data from __json_read in get_events

When QEMU process aborts and socket is closed, qmp client will not
detect it. When this happens, some qemu-iotests scripts will enter an
endless loop waiting for qmp events.

It's better we raise an exception in qmp.py to catch this and make the
test script stop.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

scripts/qmp/qmp.py

@@ -171,7 +171,12 @@ class QEMUMonitorProtocol:
                 pass
         self.__sock.setblocking(1)
         if not self.__events and wait:
-            self.__json_read(only_event=True)
+            ret = self.__json_read(only_event=True)
+            if ret == None:
+                # We are in blocking mode, if don't get anything, something
+                # went wrong
+                raise QMPConnectError("Error while reading from socket")
+
         return self.__events
 
     def clear_events(self):