Fix error in log when log exception in guestfs.py

This code review: https://review.openstack.org/#/c/104262/
brings an issue when log exception in guestfs.py at
LOG.info(_LI("Unable to force TCG mode, libguestfs too old?")
To fix this issue we just need to add %s in it.

Change-Id: Ic33a6b7c2ce49e21e96a087984aba6656065d8fc
Closes-bug: #1352768
Merge "Fix error in log when log exception in guestfs.py"