chardev: close QIOChannel before unref'ing

chardev: close QIOChannel before unref'ing

The chardev socket backend will unref the QIOChannel object while
it is still potentially open. When using TLS there could be a
pending TLS handshake taking place. If the channel is left open
then when the TLS handshake callback runs, it can end up accessing
free'd memory in the tcp_chr_tls_handshake method.

Closing the QIOChannel will unregister any pending handshake
source.

Reported-by: jiangyegen <jiangyegen@huawei.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>