bhyve: update the NVMe CQ based on the status

[chuck]

Summary

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").

This addresses concerns raised in D20914

[imp]

I am concerned no more! Awesome.

[araujo]

Lgtm!