qemu-nbd: always compile in --aio=MODE option

The --aio=MODE option enables Linux AIO or Windows overlapped I/O.

The #ifdef CONFIG_LINUX_AIO was a layering violation that also prevented
Windows overlapped I/O from being used.

Now that raw-posix.c prints an error when Linux AIO has not been
compiled in, we can unconditionally compile the option into qemu-nbd.

After this patch qemu-nbd --aio=native works on Windows.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
