destroy_vm: allow to pass new config and lock instead

destroy_vm: allow to pass new config and lock instead

This brings qemu more in line with containers, and it's nicer to
allow passing the replacement config if we want to keep it, instead
of setting a "memory: 128" config.

Use that to lock it on removal before final deletion, and on legacy
tar archive restore, in between old VM destruction and new
restoration.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>