OpenBSD doesn't use AIO so don't try to build compatfd when not using AIO.

Also make sure to call qemu_aio_init() from bdrv_init.  Everything that uses
bdrv calls bdrv_init so it makes sense to init aio from there instead of
in every single tool.
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>



git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5197 c046a42c-6fe2-441c-8c8c-71466251a162