Simplify stack setup and context initialization
- assume each vcpu has one trap stack so that trap stacks are per-vcpu
  and not per-interrupt
- add function to allocate and initialize a driver context
- add timer interrupts to vcpu setup to make interrupt masking uniform
  (no need to twiddle signal mask to block SIGALRM)
- consequently, as part of vcpu setup the user now passes hz and a trap
  trap handler for the timer interrupt (can be generic) and a trap stack
  per-vcpu
- expose PCI passthrough's simplified version of xen's shared_info,
  now named pass_status_page
- a thread link entry is allocated for the life time of the dev_pass_softc
  or the thread, so it no longer needs to be reference counted by interrupt
  allocation/deallocation