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: NHervé Poussineau <hpoussin@reactos.org>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/vvfat.c

@@ -2968,8 +2968,7 @@ vvfat_co_pwritev(BlockDriverState *bs, uint64_t offset, uint64_t bytes,
 static int64_t coroutine_fn vvfat_co_get_block_status(BlockDriverState *bs,
 	int64_t sector_num, int nb_sectors, int *n, BlockDriverState **file)
 {
-    BDRVVVFATState* s = bs->opaque;
-    *n = s->sector_count - sector_num;
+    *n = bs->total_sectors - sector_num;
     if (*n > nb_sectors) {
         *n = nb_sectors;
     } else if (*n < 0) {