nvme: add hw.nvme.min_cpus_per_ioq tunable

Due to FreeBSD system-wide limits on number of MSI-X vectors
(https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=199321),
it may be desirable to allocate fewer than the maximum number
of vectors for an NVMe device, in order to save vectors for
other devices (usually Ethernet) that can take better
advantage of them and may be probed after NVMe.

This tunable is expressed in terms of minimum number of CPUs
per I/O queue instead of max number of queues per controller,
to allow for a more even distribution of CPUs per queue.  This
avoids cases where some number of CPUs have a dedicated queue,
but other CPUs need to share queues.  Ideally the PR referenced
above will eventually be fixed and the mechanism implemented
here becomes obsolete anyways.

While here, fix a bug in the CPUs per I/O queue calculation to
properly account for the admin queue's MSI-X vector.