[arbel] Add (not-yet-functional) support for RC queue pairs

[arbel] Add (not-yet-functional) support for RC queue pairs

Arbel seems to crash the system as soon as the first send WQE
completes on an RC queue pair.  (NOPs complete successfully, so this
is a problem specific to the work queue rather than the completion
queue.)  The cause of this problem has remained unknown for over a
year.

Check in the non-functioning code to avoid bit-rot, and in the hope
that someone will find the fix.

Signed-off-by: Michael Brown <mcb30@ipxe.org>