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.
Hm, good point. Though negating the check would make the list pretty long I guess. As it would require including all the "supported" ubuntu and centos releases.

I'll try to rework it somehow..