Reduce the log level for the guestfs being missing.

There are platforms where we can never reasonably expect libguestfs
to be functional (FreeBSD, libvirt / lxc, etc). Reduce the log
level of the message recording that libguestfs is unavailable to
reduce operator confusion.

As discussed on openstack-dev at:

  http://lists.openstack.org/pipermail/openstack-dev/2014-September/046910.html

Change-Id: I4bd7ed4fbffb8ac22aea4285fe74ee0d78e1581c
Merge "Reduce the log level for the guestfs being missing."