shared/cfg/machines.cfg: Fix incorrect indentation

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.qemu_vm: change img_params to image_params

The later spelling is more clear, and used in more places
inside the code. This fixes a hidden problem in the command
creation code.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>