chardev: update net listener gcontext

chardev: update net listener gcontext

TCP chardevs can be using QIO network listeners working in the
background when in listening mode.  However the network listeners are
always running in main context.  This can race with chardevs that are
running in non-main contexts.

To solve this, we need to re-setup the net listeners in
tcp_chr_update_read_handler() with the newly cached gcontext.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180306053320.15401-4-peterx@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>