crypto: Allow gracefully ending the TLS session

crypto: Allow gracefully ending the TLS session

QEMU's TLS session code provides no way to call gnutls_bye() to
terminate a TLS session. Callers of qcrypto_tls_session_read() can
choose to ignore a GNUTLS_E_PREMATURE_TERMINATION error by setting the
gracefulTermination argument.

The QIOChannelTLS ignores the premature termination error whenever
shutdown() has already been issued. This was found to be not enough for
the migration code because shutdown() might not have been issued before
the connection is terminated.

Add support for calling gnutls_bye() in the tlssession layer so users
of QIOChannelTLS can clearly identify the end of a TLS session.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>