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>