kvm: prevent creating bad hard drive with gunzip option

slapos/recipe/kvm/template/kvm_run.in

@@ -133,9 +133,14 @@ if not os.path.exists(disk_path) and virtual_hard_drive_url != '':
   else:
     print('Warning: not checksum specified.')
   if downloaded_disk.endswith('.gz'):
+    try:
       with open(disk_path, 'w') as disk:
         with gzip.open(downloaded_disk, 'rb') as disk_gz:
           shutil.copyfileobj(disk_gz, disk)
+    except Exception:
+      if os.path.exists(disk_path):
+        os.remove(disk_path)
+      raise
     os.remove(downloaded_disk)
 
 # Create disk if doesn't exist