bootstrap: Give step based installs/setup priority for qemu

Step based installs are a qemu feature, used to install
guests, so the step.cfg file should be given priority
in qemu sets.

With this patch, they'll have priority, together with
unattended_install in testing order.

CC: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
qemu/tests/steps.py: Handle IOErrors during step engine replay

As we now start the vm paused in virt tests, it might
be a little while (some seconds) before the screendump
commands actually produce a screendump. So let's handle
IOErrors (file does not exist) when reading from the
screendump ppm file gracefully, just the same way we
do with other failures in the main test loop.

CC: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>