virt tests: Introduce inactivity watcher for unattended_install

With this patch, if the screen got inactive during unattended
install for more than 30 minutes, an error will be thrown. For
any other tests, we'll just log a debug message, since in other
tests we do not expect to see much action going on the screen.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>