Not all chardevs are created via qmp_chardev_open_socket(), and those
should not call the yank function registration, as this will eventually
assert() not being registered.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210204105232.834642-20-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>