Add support for different output formats:
1. The output file that was previously written is now called the raw format.
2. Add the vmdk output format to create VMDK images.
When the format is not given, the raw output format is assumed.
sector granularity for both offset and length. Have all schemes
use mkimg_write() instead of mkimg_seek() followed by write(2).
Now that schemes don't use lseek(2) nor write(2) directly, it's
easier to support output formats other than raw disks.
