Workaround missing by-path links for virtio disk

Newer udev releases (e.g. on SLE 12) don't create the by-path links anymore for
virtio disks. Try to workaround that for the case when the SLES11 based
discovery image tries to claim a the boot disk for a system that's going to be
installed with SLES12.
But actually just listing the 6 platforms (ubuntu-12.04, redhat-6.2, redhat-6.4, centos-6.2, centos-6.4 and suse-11.3) that should continue using the old mapping (for backwards compatibility) and have every new platform not use by-path for virtio doesn't seem to be too bad a solution.
Even when there might come a newer platform that still maintains those by-path links for virtio, their use is generally discouraged it seems.
Additionally I think I made the ```if``` close too complex I think the check for  the ```hardware-installing``` state is completely unneeded.