qemu.test: Update stg_params for (usb_)multi_disk test

Remove all the old style of stg params.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests: multi_disk: Put test steps into sub functions

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests: Add parallel test for multi_disk

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests: Add config for usb storage parallel test

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>