test/vhost: use full path in spdk_vhost.sh

Current relative work dir path caused problems with downstream
test scripts when running spdk_vhost.sh using relative path

Change-Id: Ifd47bd37650a6328cb0f548a03cb4de010093320
Signed-off-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-on: https://review.gerrithub.io/392744


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Paweł Niedźwiecki <pawelx.niedzwiecki@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>