initial attempt at rpm-ostree detection

We want to use Factory to build rpm-ostree based disk images to
support project Atomic:

http://www.projectatomic.io/

At present, libguestfs OS detection is not rpm-ostree aware.  This
adds rpm-ostree detection code in the Factory utility functions for
mounting up a disk image and returning a workable guestfs handle.

NOTE: This uses an undocumented debug API into libguestfs that
allows us to directly manipulate the utility VM.  The resulting
guestfs environment should be used read-only.  Manipulations will
almost certainly break the rpm-ostree environment.