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>