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
cleanup unused variables

- turn is_admin into a helper and use that instead
- level :info is the default log level
Merge pull request #1132 from mapleoin/cleanup

cleanup unused variables
Fix typo in variable name

undefined local variable or method `virtio_by_path_platform'
Merge pull request #1133 from dirkmueller/syntax_error_fix

Fix typo in variable name