vvfat: Fix size of temporary qcow file

The size of the qcow size was calculated so that only the FAT partition
would fit on it, but not the whole disk. However, offsets relative to
the whole disk are used to access it, so increase its size to be large
enough for that.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20211209151815.23495-1-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>