char: update read handler in all cases

In commit ac1b84dd1 (rhbz#1027181), a check was added to only update the
"read handler" when the front-end is opened, because the read callbacks
were not restored when a device is plugged. However, this seems not
correct, the handler is correctly set back on hotplug (in
virtconsole_realize) and the bug can no longer be reproduced.

Calling chr_update_read_handler() allows to fix the mux driver to stop
calling the child handlers (which may be going to be destroyed).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161003094704.18087-2-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
