pve7to8: add check for nvidia-vgpu-mgr

pve7to8: add check for nvidia-vgpu-mgr

Currently the nvidia vgpu host driver (15.2) does not support kernels >
6.0 and thus will not work with bookworm based releases for now.

Fail when the service is running, and warn if it only exists, but is
disabled/stopped (in case a user installed it sometime but did not need
it and disabled it).

In any case, link to the known issues section in the upgrade guide
(which we can update to contain up-to-date information).

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