nvme: do not revert o single I/O queue when per-CPU queues not possible

Previously nvme(4) would revert to a signle 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.

MFC after: 3 days
Sponsored by: Intel