Make sbd work with partitions too, not just whole disks

The disk claiming bit was restricting usage of devices so that it only
worked for whole disks. Now, if we get a partition, then we first
identify the disk the partition is on and then try to claim that disk.

https://bugzilla.novell.com/show_bug.cgi?id=875843
Merge pull request #98 from vuntz/sbd-partitions

Make sbd work with partitions too, not just whole disks