vvfat: Fix partition table

Unless parameter ":floppy:" is given, vvfat creates a virtual image
with DOS MBR defining a single partition which holds the FAT file
system.  The size of the virtual image depends on the width of the
FAT: 32 MiB (CHS 64, 16, 63) for 12 bit FAT, 504 MiB (CHS 1024, 16,
63) for 16 and 32 bit FAT, leaving (64*16-1)*63 = 64449 and
(1024*16-1)*64 = 1032129 sectors for the partition.

However, it screws up the end of the partition in the MBR:

    FAT width param.  start CHS  end CHS     start LBA  size
        :32:          0,1,1      1023,14,63       63    1032065
        :16:          0,1,1      1023,14,55       63    1032057
        :12:          0,1,1        63,14,55       63      64377

The actual FAT file system nevertheless assumes the partition has
1032129 or 64449 sectors.  Oops.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
