chardev: Add websocket support

New option "websocket" added to allow using WebSocket protocol for
chardev socket backend.
Example:
    -chardev socket,websocket,server,id=...

Signed-off-by: Julia Suvorova <jusual@mail.ru>
Message-Id: <20181018223501.21683-3-jusual@mail.ru>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

qapi/char.json

@@ -251,6 +251,8 @@
 #          sockets (default: false)
 # @tn3270: enable tn3270 protocol on server
 #          sockets (default: false) (Since: 2.10)
+# @websocket: enable websocket protocol on server
+#           sockets (default: false) (Since: 3.1)
 # @reconnect: For a client socket, if a socket is disconnected,
 #          then attempt a reconnect after the given number of seconds.
 #          Setting this to zero disables this function. (default: 0)
@@ -265,6 +267,7 @@
                                      '*nodelay'   : 'bool',
                                      '*telnet'    : 'bool',
                                      '*tn3270'    : 'bool',
+                                     '*websocket' : 'bool',
                                      '*reconnect' : 'int' },
   'base': 'ChardevCommon' }