qemu.tests.cpu_hotplug: Fix timeout issue and typo fix

Test may timeout when check guest's cpu after hotplug.
Update the timeout = 60 + total_cpus.

Update the log message when max cpu reached.
Set autotest case timeout to 2000s in add_max_cpu subcase.
run_autotest always timeout in add_max_cpu case.

Signed-off-by: Feng Yang <fyang@redhat.com>