qmp: add new event "command-dropped"

This event will be emitted if one QMP command is dropped.  Also,
declare an enum for the reasons.

Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180309090006.10018-16-peterx@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: rebase to master]
Signed-off-by: Eric Blake <eblake@redhat.com>

qapi/misc.json

@@ -3390,3 +3390,40 @@
 #
 ##
 { 'command': 'query-sev-capabilities', 'returns': 'SevCapability' }
+
+##
+# @CommandDropReason:
+#
+# Reasons that caused one command to be dropped.
+#
+# @queue-full: the command queue is full. This can only occur when
+#              the client sends a new non-oob command before the
+#              response to the previous non-oob command has been
+#              received.
+#
+# Since: 2.12
+##
+{ 'enum': 'CommandDropReason',
+  'data': [ 'queue-full' ] }
+
+##
+# @COMMAND_DROPPED:
+#
+# Emitted when a command is dropped due to some reason.  Commands can
+# only be dropped when the oob capability is enabled.
+#
+# @id: The dropped command's "id" field.
+#
+# @reason: The reason why the command is dropped.
+#
+# Since: 2.12
+#
+# Example:
+#
+# { "event": "COMMAND_DROPPED",
+#   "data": {"result": {"id": "libvirt-102",
+#                       "reason": "queue-full" } } }
+#
+##
+{ 'event': 'COMMAND_DROPPED' ,
+  'data': { 'id': 'any', 'reason': 'CommandDropReason' } }