aio / timers: remove dummy_io_handler_flush from tests/test-aio.c

Remove dummy_io_handler_flush from tests/test-aio.c as it does
nothing now.
aio / timers: fix build of test/test-aio.c on non-linux platforms

tests/test-aio.c used pipe2 which is Linux only. Use qemu_pipe
and qemu_set_nonblock for portabillity. Addition of O_CLOEXEC
is a harmless bonus.