windows: don't use LOG.exception if not logging an exception

windows: don't use LOG.exception if not logging an exception

There is a bug in python 3.4 such that if you call LOG.exception and
you're not in a logging handler context it fails. That bug is fixed
inadvertantly in python 3.5 under:

http://bugs.python.org/issue17911

This fixes the instance in the windows vhdutils module to use LOG.error
rather than LOG.exception to avoid that logging error.

Since LOG.exception was being used I'm assuming the original author wanted
traceback given this is a utility method, so we use the exc_info kwarg which
will log a traceback if there is one.

Closes-Bug: #1505240

Change-Id: Ib3113f2c4752d37e890f97d259da5d51cbfcfb96