uuid: Fix coverity warning of unchecked return value

Coverity checks for patterns of handling return values of functions.
Some recent addition must have tripped a threshold where coverity now
complains that we usually check the return value of virUUIDGenerate but
don't do it in one place. Add a check to make coverity happy.

tests/qemuxml2argvtest.c

@@ -51,7 +51,9 @@ fakeSecretLookupByUsage(virConnectPtr conn,
     if (STRNEQ(usageID, "mycluster_myname"))
         return NULL;
 
-    virUUIDGenerate(uuid);
+    if (virUUIDGenerate(uuid) < 0)
+        return NULL;
+
     return virGetSecret(conn, uuid, usageType, usageID);
 }