qapi: add support for mice with extra/side buttons

Adds "side" and "extra" values to enum InputButton. The naming was borrowed
from evdev since it is more descriptive than "button4" and "button5".
Signed-off-by: NFabian Lesniak <fabian@lesniak-it.de>
Message-id: 20161206190007.7539-2-fabian@lesniak-it.de
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

qapi-schema.json

@@ -5390,10 +5390,15 @@
 #
 # Button of a pointer input device (mouse, tablet).
 #
+# @side: front side button of a 5-button mouse (since 2.9)
+#
+# @extra: rear side button of a 5-button mouse (since 2.9)
+#
 # Since: 2.0
 ##
 { 'enum'  : 'InputButton',
-  'data'  : [ 'left', 'middle', 'right', 'wheel-up', 'wheel-down' ] }
+  'data'  : [ 'left', 'middle', 'right', 'wheel-up', 'wheel-down', 'side',
+  'extra' ] }
 
 ##
 # @InputAxis: