Move from a request array approach back to allocation.

virtio_blk pre-allocates requests into a cache to avoid re-allocation
(possibly an unneeded optimization with the current allocator).  However,
it doesn't take into account that requests can be completed out-of-order,
and simply reuses requests in a cyclic order. Noted by Avi although
I had it made using a peak into the index ring but its too complex
solution. There is no performance degradation w/ smp due to the good
allocator we have today.