Remove dependency on kpartx for fake RAID support

With this patch the commands required to support motherboard BIOS
RAID, also known as fake RAID, are dmraid and dmsetup.

If the kpartx command is available, it will be used to maintain
device entries that follow the naming convention used by kpartx.

Closes Ubuntu launchpad bug 554582 - GParted does not list dmraid
devices
https://bugs.launchpad.net/ubuntu/+source/gparted/+bug/554582