Prevent DiskStartSector values below the global default

Since the DiskAlignment value is used to calculate the DiskStartSector
it is possible to have a too small value.
To prevent this the global DiskStartSector default is used if the
DiskAlignment value is smaller.

This behavior can be overwritten by using the parameter
--disk-start-sector.
Merge pull request #381 from openSUSE/prevent_too_small_start_sectors

Prevent DiskStartSector values below the global default