Quote client identity in SASL whitelist log message

When seeing a message

 virNetSASLContextCheckIdentity:146 : SASL client admin not allowed in whitelist

it isn't immediately obvious that 'admin' is the identity
being checked. Quote the string to make it more obvious
(cherry picked from commit 07da0a6b)

src/rpc/virnetsaslcontext.c

@@ -163,7 +163,7 @@ int virNetSASLContextCheckIdentity(virNetSASLContextPtr ctxt,
     }
 
     /* Denied */
-    VIR_ERROR(_("SASL client %s not allowed in whitelist"), identity);
+    VIR_ERROR(_("SASL client identity '%s' not allowed in whitelist"), identity);
 
     /* This is the most common error: make it informative. */
     virReportError(VIR_ERR_SYSTEM_ERROR, "%s",