QMP: BLOCK_IO_ERROR event handling

This commit adds the basic definitions for the BLOCK_IO_ERROR
event, but actual event emission will be introduced by the
next commits.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/monitor.h b/monitor.h
index b0f9270..e35f1e4 100644
--- a/monitor.h
+++ b/monitor.h
@@ -23,6 +23,7 @@
     QEVENT_VNC_CONNECTED,
     QEVENT_VNC_INITIALIZED,
     QEVENT_VNC_DISCONNECTED,
+    QEVENT_BLOCK_IO_ERROR,
     QEVENT_MAX,
 } MonitorEvent;