pr-manager-helper: report event on connection/disconnection

Let management know if there were any problems communicating with
qemu-pr-helper.  The event is edge-triggered, and is sent every time
the connection status of the pr-manager-helper object changes.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qapi/block.json

@@ -358,6 +358,30 @@
 { 'event': 'DEVICE_TRAY_MOVED',
   'data': { 'device': 'str', 'id': 'str', 'tray-open': 'bool' } }
 
+##
+# @PR_MANAGER_STATUS_CHANGED:
+#
+# Emitted whenever the connected status of a persistent reservation
+# manager changes.
+#
+# @id: The id of the PR manager object
+#
+# @connected: true if the PR manager is connected to a backend
+#
+# Since: 3.0
+#
+# Example:
+#
+# <- { "event": "PR_MANAGER_STATUS_CHANGED",
+#      "data": { "id": "pr-helper0",
+#                "connected": true
+#      },
+#      "timestamp": { "seconds": 1519840375, "microseconds": 450486 } }
+#
+##
+{ 'event': 'PR_MANAGER_STATUS_CHANGED',
+  'data': { 'id': 'str', 'connected': 'bool' } }
+
 ##
 # @QuorumOpType:
 #