When attaching the NBD QIOChannel to an AioContext, the TLS channel should
be used, not the underlying socket channel.  This is because, trivially,
the TLS channel will be the one that we read/write to and thus the one
that will get the qio_channel_yield() call.

Fixes: ff82911c
Cc: qemu-stable@nongnu.org
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NDaniel P. Berrange <berrange@redhat.com>
Tested-by: NDaniel P. Berrange <berrange@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>