See also:
http://libguestfs.org/guestfs.3.html#inspection
"The call "guestfs_inspect_os" performs inspection and caches the
results in the guest handle. Subsequent calls to guestfs_inspect_get_*
return this cached information, but do not re-read the disks. If you
change the content of the guest disks, you can redo inspection by
calling "guestfs_inspect_os" again."
Rich.
--
Richard Jones, Virtualization Group, Red Hat
http://people.redhat.com/~rjones
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine. Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/