storage: Break out the loop if duplicate pool is found

It doesn't break out the "for" loop even if duplicate pool is
found, and thus the "matchpool" could be overriden as NULL again
if there is different pool afterwards.

To address the problem in libvirt-user list:

https://www.redhat.com/archives/libvirt-users/2012-April/msg00150.html
(cherry picked from commit 980f12be)

src/conf/storage_conf.c

@@ -1710,6 +1710,9 @@ int virStoragePoolSourceFindDuplicate(virStoragePoolObjListPtr pools,
             break;
         }
         virStoragePoolObjUnlock(pool);
+
+        if (matchpool)
+            break;
     }
 
     if (matchpool) {