by-path links for virtio disk might not exist

Newer udev releases don't create the by-path links anymore for virtio disks.
Don't try to resolve to "by-path" links for virtio devices for any platform
that is newer than what we currenlty support (e.g. SLE-12). Use the /dev/vdX
names in that case instead.
Merge pull request #1129 from rhafer/sle-12-virtio

by-path links for virtio disk might not exist