tape: library status: don't fail if the library does not support DVCID

tape: library status: don't fail if the library does not support DVCID

some libraries (e.g. Qualstar) don't support the DVCID bit in the READ
ELEMENT (B8) command (to return vendor/model of connected drives), so
make that part optional if it fails. We only ever use the serial number
in the `pmtx` tool, so there is not much downside to not having this.

This increases compatibility with such libraries

Reported in the forum:
https://forum.proxmox.com/threads/cant-query-tape-robot-status.131833/

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