Fix #2412: Missing VMs in pools

Fix #2412: Missing VMs in pools

Between calling vm_destroy and removing the ID from user.cfg (remove_vm_access)
creating a new VM with this ID was possible. VMs could go missing from pools as
a consequence.

Adding a lock solves this for clones from the same node. Additionally,
unlinking must happen at the very end of the deletion process to avoid that
other nodes use the ID in the meanwhile.

Co-developed-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Signed-off-by: Dominic Jäger <d.jaeger@proxmox.com>