storage: Resolve issues in failure path

https://bugzilla.redhat.com/show_bug.cgi?id=1092882

Refactoring in commit id '0c2305b3' resulted in the wrong storage
volume object being passed to the new storageVolDeleteInternal().
It should have passed 'voldef' which is the address found in the
pool->volumes.objs[i] array.  By passing 'voldef', the DeleteInternal
code will find and remove the voldef from the volumes.objs[] list.

src/storage/storage_driver.c

@@ -1766,12 +1766,11 @@ storageVolCreateXML(virStoragePoolPtr obj,
         voldef->building = 0;
         pool->asyncjobs--;
 
-        voldef = NULL;
-
         if (buildret < 0) {
-            storageVolDeleteInternal(volobj, backend, pool, buildvoldef,
+            VIR_FREE(buildvoldef);
+            storageVolDeleteInternal(volobj, backend, pool, voldef,
                                      0, false);
-            buildvoldef = NULL;
+            voldef = NULL;
             goto cleanup;
         }