vnc_server_cut_text_caps() is not guaranteed to be called only once.

If it called twice, we finally call notifier_list_add() twice with same
element. Which leads to loopback QLIST. So, on next
notifier_list_notify() we'll loop forever and QEMU stuck.

So, let's only register new notifier if it's not yet registered.

Note, that similar check is used in vdagent_chr_recv_caps() (before
call qemu_clipboard_peer_register()), and also before
qemu_clipboard_peer_unregister() call in vdagent_disconnect() and in
vnc_disconnect_finish().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211110103800.2266729-1-vsementsov@virtuozzo.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>