vvfat: limit number of entries in root directory in FAT12/FAT16

FAT12/FAT16 root directory is two sectors in size, which allows only 512 directory entries.
Prevent QEMU startup if too much files exist, instead of overflowing root directory.

Also introduce variable root_entries, which will be required for FAT32.

Fixes: https://bugs.launchpad.net/qemu/+bug/1599539/comments/4
Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
