bhyve: update the NVMe CQ based on the status

Instead of skipping the NVMe Completion Queue update based on the
opcode, define a synthetic status value which indicates the completion
queue entry is invalid. This will also allow deferred completion queue
updates for other commands.

Also returns the correct status for unrecognized opcodes ("invalid
opcode").

Reviewed by: imp, jhb, araujo
Approved by: imp (mentor), jhb (maintainer)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20945