conf: Fix memory leaks in virStoragePoolDefParseSource

Detected by valgrind. Leaks are introduced in commit 122fa379.

src/conf/storage_conf.c: fix memory leaks.

How to reproduce?
$ make && make -C tests check TESTS=storagepoolxml2xmltest
$ cd tests && valgrind -v --leak-check=full ./storagepoolxml2xmltest

actual result:
==28571== LEAK SUMMARY:
==28571==    definitely lost: 40 bytes in 5 blocks
==28571==    indirectly lost: 0 bytes in 0 blocks
==28571==      possibly lost: 0 bytes in 0 blocks
==28571==    still reachable: 1,054 bytes in 21 blocks
==28571==         suppressed: 0 bytes in 0 blocks
Signed-off-by: NAlex Jia <ajia@redhat.com>

src/conf/storage_conf.c

@@ -465,6 +465,7 @@ virStoragePoolDefParseSource(xmlXPathContextPtr ctxt,
         }
     }
 
+    VIR_FREE(nodeset);
     source->initiator.iqn = virXPathString("string(./initiator/iqn/@name)", ctxt);
 
     nsource = virXPathNodeSet("./device", ctxt, &nodeset);