ui/vnc: clipboard support

This patch adds support for cut+paste to the qemu vnc server, which
allows the vnc client exchange clipbaord data with qemu and other peers
like the qemu vdagent implementation.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20210519053940.1888907-1-kraxel@redhat.com
Message-Id: <20210519053940.1888907-8-kraxel@redhat.com>