Fix leak in SCSI storage backend

The SCSI storage backend leaks a string containing the pathname
for each block device it discovers

* src/storage/storage_backend_scsi.c: Free the device name

src/storage/storage_backend_scsi.c

@@ -455,6 +455,7 @@ processLU(virStoragePoolObjPtr pool,
     VIR_FREE(type_path);
 
 out:
+    VIR_FREE(block_device);
     return retval;
 }