This is a WIP, not intended to be committed as-is.
Implement vcpu_lock_all() in a way that avoids deadlocks with
vm_smp_rendezvous(). In particular, when traversing vCPUs, invoke the
rendezvous callback on the vCPU's behalf to help the initiator finish.
We can only safely do so when the vCPU is IDLE or we have already locked
it.