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 <[email protected]>
Cc: Laurent Vivier <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Wainer dos Santos Moschetta <[email protected]>
[thuth: Replaced global_qtest with local qts variable]
Signed-off-by: Thomas Huth <[email protected]>