fix #5229: tape: remove max sequence number limit

fix #5229: tape: remove max sequence number limit

The idea was to limit the number of tapes in a media set, but this was
not enforced when adding a medium to a media set, only on read/parsing
the inventory. With that, it is possible to create media sets greater
than the limit which in turn blocks access to most functions via
api/cli/gui due to the check.

Instead of enforcing an arbitrary limit, simply warn on creation when
the media-set is very large (20).

To restore the whole media set, the time taken would still be at least 38
hours for LTO-4 and 250 hours for LTO-9.

We already have a section in the docs where we tell about the
disadvantages of large media sets.

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