Generate facts for device labels, UUID, and ids; stop excluding virtual devices

At present, the available facts around block devices are not sufficient to be able to find stable names guaranteed to work across reboots, or to identify block devices by label (UUID, etc).

This patch provides a list of observed links for each device. It relies on functionality specific to Linux (as does the existing sysfs-based code which it extends), but should not cause issues on other platforms.