move from deferring vd requests with a task per message to a ring

move from deferring vd requests with a task per message to a ring
of pointers to vd requests, and a single task to notify the taskq
that there's work on the ring.

the ring is modelled on what you usually get with network cards,
which is an array with a producer and consumer index. the interrupt
handler fills an entry in the ring at the producers index and
increments it. the task reads entries at the consumer index and
increments it until it reaches the same value as the producer.

tested and slightly tweaked by kettenis@
ok kettenis@