Switch to GSSAPI (kerberos) instead of the insecure DIGEST-MD5
    
    RFC 6331 documents a number of serious security weaknesses in
    the SASL DIGEST-MD5 mechanism. As such, libvirtd should not
    by using it as a default mechanism. GSSAPI is the only other
    viable SASL mechanism that can provide secure session encryption
    so enable that by defalt as the replacement.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>