nbd: fix NBD over TLS

nbd: fix NBD over TLS

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: ff82911cd3f69f028f2537825c9720ff78bc3f19
Cc: qemu-stable@nongnu.org
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Tested-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>