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>

qemu.sasl

@@ -22,7 +22,9 @@ mech_list: digest-md5
 # Some older builds of MIT kerberos on Linux ignore this option &
 # instead need KRB5_KTNAME env var.
 # For modern Linux, and other OS, this should be sufficient
-keytab: /etc/qemu/krb5.tab
+#
+# There is no default value here, uncomment if you need this
+#keytab: /etc/qemu/krb5.tab
 
 # If using digest-md5 for username/passwds, then this is the file
 # containing the passwds. Use 'saslpasswd2 -a qemu [username]'