nvme: fall back to a smaller MSI-X vector allocation if necessary

Previously, if per-CPU MSI-X vectors could not be allocated,
nvme(4) would fall back to INTx with a single I/O queue pair.
This change will still fall back to a single I/O queue pair, but
allocate MSI-X vectors instead of reverting to INTx.

MFC after: 1 week
Sponsored by: Intel