nbd: enable use of TLS with NBD block driver

nbd: enable use of TLS with NBD block driver

This modifies the NBD driver so that it is possible to request
use of TLS. This is done by providing the 'tls-creds' parameter
with the ID of a previously created QCryptoTLSCreds object.

For example

  $QEMU -object tls-creds-x509,id=tls0,endpoint=client,\
                dir=/home/berrange/security/qemutls \
        -drive driver=nbd,host=localhost,port=9000,tls-creds=tls0

The client will drop the connection if the NBD server does not
provide TLS.

Signed-off-by: Daniel P. Berrange <[email protected]>
Message-Id: <1455129674[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>