tape: changer: correctly consume data in decode_element_status_page

tape: changer: correctly consume data in decode_element_status_page

instead of 'blindly' trusting the changer to deliver the fields written
in the specification, trust the length data it returns in the header.

we slice the descriptor data into equal sized chunks of the correct
size, then we do not have care bout the len and empty checks anymore

this also makes the code to read the rest of the page obsolete,
since the next descriptor is on the correct offset anyway

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