input: qapi: add unmapped key

Simplifies building something -> QkeyCode mapping tables.
Uninitialized entries can easily identified then.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

qapi-schema.json

@@ -3555,9 +3555,12 @@
 # This is used by the send-key command.
 #
 # Since: 1.3.0
+#
+# 'unmapped' since 2.0
 ##
 { 'enum': 'QKeyCode',
-  'data': [ 'shift', 'shift_r', 'alt', 'alt_r', 'altgr', 'altgr_r', 'ctrl',
+  'data': [ 'unmapped',
+            'shift', 'shift_r', 'alt', 'alt_r', 'altgr', 'altgr_r', 'ctrl',
             'ctrl_r', 'menu', 'esc', '1', '2', '3', '4', '5', '6', '7', '8',
             '9', '0', 'minus', 'equal', 'backspace', 'tab', 'q', 'w', 'e',
             'r', 't', 'y', 'u', 'i', 'o', 'p', 'bracket_left', 'bracket_right',