nvme: do not revert to single I/O queue when per-CPU queues not available

Previously nvme(4) would revert to a single I/O queue if it could not
allocate enought interrupt vectors or NVMe submission/completion queues
to have one I/O queue per core.  This patch determines how to utilize a
smaller number of available interrupt vectors, and assigns (as closely
as possible) an equal number of cores to each associated I/O queue.