Fix bhyve's NVMe Completion Queue entry values

The function which processes Admin commands was not returning the
Command Specific value in Completion Queue Entry, Dword 0 (CDW0). This
effects commands such as Set Features, Number of Queues which returns
the number of queues supported by the device in CDW0. In this case, the
host will only create 1 queue pair (Number of Queues is zero based).
This also masked a bug in the queue counting logic.

Reviewed by: imp, araujo
Approved by: imp (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D18703