The VM and compute claim decorators checked only that the user sees the
VM, took the claim, and left the action's own right to the view inside:
a read-only viewer, or an owner who may not delete the VM, briefly held
the VM's or the whole compute's claim and could make other requests
there busy. They now check the least right first: power for a VM change,
delete for the compute-wide claim (destroy; its other views are
superuser-only).
