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>
virttest.env_process: Avoid autotest hang in postprocess

When process fails in postprocess, screendump thread is not killed
and autotest hangs forever.

Additionally continue with postprocess when some part fails and reraise
the exception at the end.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests.hdparm: Fix issue that start VM twice

Now hdparm will start VM twice.
hdparm no need special setup before starting VM.
So only start VM in framework.

Signed-off-by: Feng Yang <fyang@redhat.com>