Check that at least 2 disks are selected for raid

When configuring RAID, 2 disks are at least needed. Otherwise i.e. the
autoyast.xml template gets wrong values and the installation aborts.
Hrm, RAID 5 needs 3 disks, iirc. Don't know about RAID 6 or RAID 10; but in short, the test is a bit more complex :-)