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.

Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>