tape: work around buggy changer implementations

tape: work around buggy changer implementations

allocation length for read element status is a 3 byte field, but it
seems some changers only look at the bottom two bytes. Since we used
0x010000 for it, those changers did not return any data and the calls
failed.

To work around it, request one byte less (0xFFFF) which should still be
enough for the data, but should now work with those buggy
implementations.

Reported by a user in the forum: https://forum.proxmox.com/threads/137391/

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>