char: remove watch callback on chardev detach from frontend

If a frontend device releases the chardev (via unplug), the chr handlers
are set to NULL via qdev's exit callbacks invoking
qemu_chr_add_handlers().  If the chardev had a pending operation, a
callback will be invoked, which will try to access data in the
just-released frontend, causing a segfault.

Ensure the callbacks are disabled when frontends release chardevs.

This was seen when a virtio-serial port was unplugged when heavy
guest->host IO was in progress (causing a callback to be registered).
In the window in which the throttling was active, unplugging ports
caused a qemu segfault.

https://bugzilla.redhat.com/show_bug.cgi?id=985205

CC: <qemu-stable@nongnu.org>
Reported-by: Sibiao Luo <sluo@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
