fixes for cpu_hotplug_test

- change echo command from virttest/aexpect.py cmd_output function (did not seem
  to work as written. The original command is jumbled in the output, and
  the string comparison does not work, resulting in the command staying in
  the output buffer instead of being removed)

- fix sorting in /sys/devices/system/cpu/cpu[0-99]/online files. list.sort() in
  python returns none, so the original code assigned none to the output list,
  resulting in unnecessary testcase failure.
add fc18 in guest-os.cfg

Also add fc18 and fc17 in tests.cfg
add debian pb image config