lvmstrap: add PV-on-partition support

This is a not-so-nice change, adding support for partitions to be used
as PVs.

The not-nice part is that partitions live in a separate place in
sysfs, whereas in dev they live at the same level as disks. We
workaround this via a new SysfsName function that computes the correct
sysfs base path for a given disk.

The other rule is that if a disk is not in use, we ignore its
partitions completely, as the disk will be re-partitioned anyway. Only
if the disk is busy, we consider each of its partitions for the
free/busy list.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>