nvme: name the Create I/O queue cdw11 fields, use Medium QPRIO

The Create I/O queue commands carried magic 0x1/0x3 cdw11 values, and
the implicit QPRIO of zero classifies every I/O queue Urgent the
moment weighted round robin arbitration is enabled. No functional
change with the round robin default.

Approved by: ngie (co-mentor)

Reviewed by: ngie, imp, adrian
Differential Revision: https://reviews.freebsd.org/D59153