ao_how: worst result. no domain_suspend_cb and domain_destroy_cb
add ao_how callback. NOT TEST
test libxl_event_check. TEST FAIL
add libxl_sigchild_callback and libxl_fork_replacement
add ao_how in libxlVmReap
could not enter libxl_childproc_reaped stwice for the same fd(same pid)
fix wrong logic in libxlVmReap after ao_how_enable_cb
modify for ao_how with callbck, NOT COMPILE