storage: plug iscsi memory leak

Detected by Coverity.  virStoragePoolSourceFree does not free the
actual passed-in pointer.  A bigger patch would be to rename it
virStoragePoolSourceClear to match behavior, or even split it into
two functions depending on needed behavior; but this is the minimal
fix to the one location out of eight that leaked memory.

* src/storage/storage_backend_iscsi.c
(virStorageBackendISCSIFindPoolSources): Free memory.

src/storage/storage_backend_iscsi.c

@@ -611,6 +611,7 @@ cleanup:
     VIR_FREE(targets);
     VIR_FREE(portal);
     virStoragePoolSourceFree(source);
+    VIR_FREE(source);
     return ret;
 }