fix #4904: tape changer: add option to eject before unload

fix #4904: tape changer: add option to eject before unload

some tape libraries need the tape being ejected from the drive before
doing an unload. Since we cannot easily detect if that's the case,
introduce an 'eject_before_unload' option.

Instead of just adding a bool flag to the config, add a new 'options'
property string where we can put such niche options similar to how we
handle the datastore tuning options.

Extend the LtoTapeHandle with 'medium_present' which just uses a
TEST UNIT READY command to check for present medium, so we don't
try to eject an already ejected tape.

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