Correct import in lvhd-thin plugin

Signed-off-by: Damien Thenot <damien.thenot@vates.tech>

Author: Nambrok

drivers/lvhd-thin

@@ -21,7 +21,7 @@
 import sys
 import XenAPIPlugin
 sys.path.append("/opt/xensource/sm/")
-import lvmcowutil
+from lvmcowutil import LvmCowUtil, getCowUtil
 import util
 from lvmcache import LVMCache
 from journaler import Journaler