fix invalid device passthrough being added to config

fix invalid device passthrough being added to config

Fix a bug that allows a device passthrough entry to be added to the
config despite the device path not pointing to a device. Previously,
adding an invalid device passthrough entry would throw an error, but the
entry would still be added to the config. This is fixed by moving the
respective checks from update_lxc_config to update_pct_config, which is
run before the entry is written to the config file.

Signed-off-by: Filip Schauer <f.schauer@proxmox.com>
[FE: drop hunk for use statements left-over from earlier version]
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>