monitor: Create QAPIfied monitor_init()

This adds a new QAPI-based monitor_init() function. The existing
monitor_init_opts() is rewritten to simply put its QemuOpts parameter
into a visitor and pass the resulting QAPI object to monitor_init().

This will cause some change in those error messages for the monitor
options in the system emulator that are now generated by the visitor
rather than explicitly checked in monitor_init_opts().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200224143008.13362-17-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qapi/control.json

@@ -216,3 +216,39 @@
 # <- { "return": {} }
 ##
 { 'command': 'quit' }
+
+##
+# @MonitorMode:
+#
+# An enumeration of monitor modes.
+#
+# @readline: HMP monitor (human-oriented command line interface)
+#
+# @control: QMP monitor (JSON-based machine interface)
+#
+# Since: 5.0
+##
+{ 'enum': 'MonitorMode', 'data': [ 'readline', 'control' ] }
+
+##
+# @MonitorOptions:
+#
+# Options to be used for adding a new monitor.
+#
+# @id:          Name of the monitor
+#
+# @mode:        Selects the monitor mode (default: readline)
+#
+# @pretty:      Enables pretty printing (QMP only)
+#
+# @chardev:     Name of a character device to expose the monitor on
+#
+# Since: 5.0
+##
+{ 'struct': 'MonitorOptions',
+  'data': {
+      '*id': 'str',
+      '*mode': 'MonitorMode',
+      '*pretty': 'bool',
+      'chardev': 'str'
+  } }