bhyve: fix NVMe emulation update of SQHD

The SQHD field of a Completion Queue entry indicates the current
Submission Queue head pointer value. The head pointer represents the
next entry to be consumed and is updated after consuming the current
entry.

In the Admin queue processing, the current code updates the head pointer
after reporting the value to the host via the SQHD. This gives the
impression that the Controller is perpetually one command behind in its
processing of the Admin SQ. And while this doesn't appear to bother some
initiators, it is wrong.

Fix is to update the SQ head pointer prior to writing the SQHD value in
the completion.

While here, fix missed update of dword 0 (cdw0) in the completion
message.

Reported by: khng300
Reviewed by: jhb, imp
Approved by: jhb (maintainer)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24083