web: always allow create_vm under NetBox-declared VMID when guest absent
NetBox is authoritative: a vm-rack device with a declared VMID and no live Proxmox guest now always offers 'Create ... as VMID ' instead of blocking with BlockerVMIDMissingOnProxmox. Fixes device 'screen' (VMID 297) being un-creatable after Rax had recorded a provisioned row. Tests updated: the blocks-when-provisioned test is replaced with an allows-even-after-provisioned test.