by-path links for virtio disk might not exist

Newer udev releases (e.g. on SLE 12) don't create the by-path links anymore for
virtio disks. Additionally they are not stable according to:
https://www.mail-archive.com/systemd-devel@lists.freedesktop.org/msg20607.html
Avoid using them for all platforms newer than what we currently support. Keep
the old behavior intact on currently supported platforms for backwards
compatibility.
Merge pull request #220 from rhafer/sle-12-virtio

Workaround missing by-path links for virtio disk