recognise disks with full-disk LVM

If a disk contains nothing but an LVM volume, lshw-B.02.16 will report
it as `class="volume"` (but with `id="disk:N"). Add this as a test case,
and recognise it as a disk. Real partitions still need to be ignored.
Merge pull request #192 from soundcloud/lvm-on-blank-disk

recognise disks with full-disk LVM