On F17 at least, every time libvirtd starts we get this in syslog:

libvirtd: Could not find keytab file: /etc/libvirt/krb5.tab: No such file or directory

This comes from cyrus-sasl, and happens regardless of whether the
gssapi plugin is requested, which is what actually uses
/etc/libvirt/krb5.tab.

While cyrus-sasl shouldn't complain, we can easily make it shut up by
commenting out the keytab value by default.

Also update the keytab comment to the more modern one from qemu's
sasl config file.
(cherry picked from commit fe772f24)