vnc websocket compatibility issue

WebSocket support in VNC should allow access from VNC client through upgraded WebSocket connection. This feature is not working in IE 11/Edge with noVNC HTML5 client, in contrast to that in Firefox/Safari, etc.

The reason that IE 11/Edge fails to accept the connection upgrade is that the value equality of the `Upgrade` header field is checked in a strict case-sensitive manner in QEMU side, however, the IE/Edge does not send the exactly same string value `websocket` but a capital letter `W` instead.

Defined in section 4.2.1 of rfc6455, the upgrade header field shall be treated case-insensitively.

A patch shall be made in `io/channel-websock.c`, converting the value of upgrade string to lowercase before comparison is made with QIO_CHANNEL_WEBSOCK_UPGRADE_WEBSOCKET, to allow case-insensitive comparison in the process.