block: deprecate iSCSI 'password' in favour of 'password-secret'

Support for referencing secret objects was added in

  commit b189346e
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Thu Jan 21 14:19:21 2016 +0000

    iscsi: add support for getting CHAP password via QCryptoSecret API

The existing 'password' option is overdue for deprecation and
subsequent removal.

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