nvmf: make sure nvmf target workable when it run as secondary porcess.

This patch do the following optimization:
1 when the nvmf target run as secondary process, the nvme probe will
  not call the probe callback function, which will skip the step to
  set up the ctx->found, so we set up it when we call attach callback.
2 when the nvmf target run as secondary process, every time when we try
  to probe one device, it will iterate all the attached devices and call
  the attach callback, for nvmf target, it will cause the overwrite of
  the nvme controller for the nvmf subsystem, so we need to compare the
  attached deivce with the device which we try to attach, if they are not
  the same one, just return.

Change-Id: I606ba75b1dab7fd1b1e4e1cfcf63248a506725d1
Signed-off-by: cunyinch <cunyin.chang@intel.com>
Reviewed-on: https://review.gerrithub.io/362807


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>