test/vhost: fix variable error in vm_setup

apparently getopt used $i when iterating over variables because when
weuse --force and don't set i ourselves, bash throws errors that look
like "line 594: [[: --test-cases=1,2: attempted assignment to
non-variable (error token is "=1,2")".

Change-Id: I89937f1cdd3d3c0557c931a11707df17c7b8d87e
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/478079


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>