sasl: Avoid 'Could not find keytab file' in syslog

The "keytab" specification in "qemu.sasl" only makes sense if "gssapi" is
selected in "mech_list". Even if the latter is not done (ie. "gssapi" is
not selected), the cyrus-sasl library tries to open the specified keytab
file, although nothing has a use for it outside the gssapi backend.

Since the default keytab file "/etc/qemu/krb5.tab" is usually absent, the
cyrus-sasl library emits a warning to syslog at startup, which tends to
annoy users (who didn't ask for gssapi in the first place).

Comment out the keytab specification per default.

"qemu-doc.texi" already correctly explains how to use "mech_list: gssapi"
together with "keytab:".

See also:
- upstream libvirt commit fe772f24,
- Red Hat Bugzilla <https://bugzilla.redhat.com/show_bug.cgi?id=1018434>.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
ACKed-By: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
