fix(vmware): skip independent-nonpersistent disks to avoid backup noise

VMs backed up with tools like Veeam temporarily attach
'Independent-nonpersistent' disks during the backup window. When
netbox-sync runs at that time it adds and then immediately deletes
NetBox VirtualDisk objects, producing two unnecessary changelog entries
per backup job.

This fix detects disks with diskMode == 'independent_nonpersistent'
during the VMware inventory scan and:
1. Skips adding them to the discovered disk list.
2. Tracks their device labels in a set (skipped_independent_disks).
3. Passes that set to add_device_vm_to_inventory so any pre-existing
   NetBox entry for such a disk is marked for deletion instead of being
   compared against a new entry.

Fixes: disk churn caused by backup tooling (e.g. Veeam) attaching
independent-nonpersistent snapshot helper disks.

Author: Littlericket