Support 9p-virtio filesystem for mounting the DCS-directory.

This patch allows to use exported directories via 9p-virtio to be used as
DCS-directory. This allows customization of standard Grml ISO images in
virtualized environments without the need to adjust anything or create
specialized blockdevices/floppy images.

For a short introduction about using 9p with qemu/kvm see
http://wiki.qemu.org/Documentation/9psetup