vvfat: reorganize computation of disk geometry

First determine FAT12/16/32, then compute geometry from that for both
FDD and HDD.  For 1.44MB floppies, and 2.88MB floppies using FAT16,
change to 1 sector/cluster.  The default remains 2.88MB with FAT12
and 2 sectors/cluster.  Both DOS and mkdosfs by default format a 2.88MB
floppy as FAT12.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
