virt.virt_env_process: fail tests due to guest screen inactivity

In a very specific test (unattended_install), all OEM installer
mechanism will present intense guest screen activity, so screen
inactivity for a relatively long time almost always mean something
went very wrong and the installer is stuck.

So create a mechanism to detect screen inactivity for any vm that
takes place in the test - have a timer for each vm that is reset
only if the next screenshot is different than the previous. So,
if the vm got stuck somewhere at the install process, for example:

* Wrong Windows CD Key
* Guest OS does not see a disk controller due to lack of drivers
* BIOS got stuck

And other similar cases, we'll have a test failure before the full
test timeout (that for Windows may be up to 3 hours). So we save
time and know about such situations earlier.

Next patch introduces the parameters on base.cfg, subtests.cfg, and
also introduces a sensible default for inactivity treshold (30 min).
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

tests/unattended_install.py

@@ -828,6 +828,7 @@ def run_unattended_install(test, params, env):
             else:
                 raise e
         vm.verify_kernel_crash()
+        test.verify_background_errors()
         finish_signal = vm.serial_console.get_output()
         if (params.get("wait_no_ack", "no") == "no" and
             (post_finish_str in finish_signal)):