IH tracks pending retry faults in a hash table for fast lookup in
interrupt context. Each VM has a short FIFO of pending VM faults for
processing in a bottom half.

The IH prescreening stage adds retry faults and filters out repeated
retry interrupts to minimize the impact of interrupt storms.

It's the VM's responsibility remove pending faults once they are
handled. For now this is only done when the VM is destroyed.

v2:
- Made the hash table smaller and the FIFO longer. I never want the
  FIFO to fill up, because that would make prescreen take longer.
  128 pending page faults should be enough to keep migrations busy.
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Christian König <christian.koenig@amd.com> (v1)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>