tests: Exit boot-serial-test loop if child dies

tests: Exit boot-serial-test loop if child dies

There's no point in waiting 5 full minutes when there will be
no more output.  Compute timeout based on elapsed wall clock
time instead of N * delays, as the delay is a minimum sleep time.

Cc: Thomas Huth <thuth@redhat.com>
Cc: Laurent Vivier <lvivier@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
[thuth: Replaced global_qtest with local qts variable]
Signed-off-by: Thomas Huth <thuth@redhat.com>