vvfat: fix qemu-img map and qemu-img convert

- bs->total_sectors is the number of sectors of the whole disk
- s->sector_count is the number of sectors of the FAT partition

This fixes the following assert in qemu-img map:
qemu-img.c:2641: get_block_status: Assertion `nb_sectors' failed.

This also fixes an infinite loop in qemu-img convert.

Fixes: 4480e0f9
Fixes: https://bugs.launchpad.net/qemu/+bug/1599539


Cc: qemu-stable@nongnu.org
Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>