virttest.env_process: Process all images at once

Previously each image check paused and unpaused the VM. With this
patch it pause vm, checks all images and unpause again (if required).

Additionally this processes the all remaining images in case of
failure thus image check failure won't prevent other images to be
checked/deleted (in postprocess).

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>