Avoid using deprecated gnutls functions with recent gnutls versions.
Fixes build failure on Fedora 16.  Keep the old way for compatibility
with old installations such as RHEL-5 (gnutls 1.4.x).

Based on a patch from Raghavendra D Prabhu <raghu.prabhu13@gmail.com>

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>