fix #4372: fix vm_resume migration callback

fix #4372: fix vm_resume migration callback

the fix for the recently introduced requirement of loading the VM config while
migrating was incomplete, since the vmlist node value could already be out of
date by the time load_config is called.

extend the fallback behaviour even further, by doing the following sequence:
- try regular load_config (likely case, rename already fully processed)
- if it fails, get node from vmlist, and load_config using that
- it that fails, invalidate the PVE::Cluster cache, retry regular load_config

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>