crypto: push error reporting into TLS session I/O APIs

crypto: push error reporting into TLS session I/O APIs

The current TLS session I/O APIs just return a synthetic errno
value on error, which has been translated from a gnutls error
value. This looses a large amount of valuable information that
distinguishes different scenarios.

Pushing population of the "Error *errp" object into the TLS
session I/O APIs gives more detailed error information.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>