tape: write informational MAM attributes on tapes

tape: write informational MAM attributes on tapes

namely:

Vendor: Proxmox
Name: Backup Server
Version: current running package version
User Label Text: the label text
Media Pool: the current media pool

write it on labeling and when writing a new media-set to a tape.

While we currently don't use this info for anything, this can help users
to identify tapes, even with different backup software.

If we need it in the future, we can e.g. make decisions based on these
fields (e.g. the version).

On format, delete them again.

Note that some VTLs don't correctly delete the attributes from the
virtual tapes.

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