test/vhost: lower fio job iodepth

Temporarily lower iodepth to avoid random test pool
failures while running vhost blk fio tests.

Reenable old iodepth value once 'too high' vhost blk performance
is solved.

Change-Id: I385ff3151b4433b331807be6ce0f3103980bae7c
Signed-off-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-on: https://review.gerrithub.io/377697


Reviewed-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>