tape: move 'set_encryption' calls to the TapeDriver (and implementation)

tape: move 'set_encryption' calls to the TapeDriver (and implementation)

namely everytime we know what the key for the tape has to be:
* after we write the MediaSetLabel
* after reading the MediaSetLabel

When handling data on tape, we always have to have the MediaSetLabel, so
we should always trigger one of these. Because of that, we should not be
able to forget to set the encryption mode.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
(cherry picked from commit 1343dcaf011ee81941d9160e9f12e6c36ca69d50)
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>