vnc: add support for extended desktop resize

vnc: add support for extended desktop resize

The extended desktop resize encoding adds support for (a) clients
sending resize requests to the server, and (b) multihead support.

This patch implements (a).  All resize requests are rejected by qemu.
Qemu can't resize the framebuffer on its own, this is in the hands of
the guest, so all qemu can do is forward the request to the guest.
Should the guest actually resize the framebuffer we can notify the vnc
client later with a separate message.

This requires support in the display device.  Works with virtio-gpu.

https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#extendeddesktopsize-pseudo-encoding

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20210112134120.2031837-4-kraxel@redhat.com