ntb_perf: extend with doorbell latency measurement

Doorbell latency is a delay between start to ring an NTB doorbell
and receiving the confirmation. The remote system needs to be run in server
mode beforehand. Then the server waits for a doorbell event and immediately
rings self doorbell to confirm.

Thanks-to: Guo Zhengkui <guozhengkui@vivo.com>
Signed-off-by: Alexander Fomichev <a.fomichev@yadro.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>