stub libaio support

This patch begins to add libaio support to OSv (see issue #494).
Note that libaio is a Linux-only asynchronous I/O API, which is distinct
from the Posix aio API (and newer, and supposedly better).

The implementation in this patch is silly and trivial: It only
implements io_setup(), which does nothing except print a stubbing
warning. This is just enough to "satisfy" applications which think they
need libaio, but end up not actually using it in practice. This patch can
also serve as a starting point for more serious work on libaio support in
OSv.

If the application attempts to call one of the other io_*() functions
after io_setup(), it will find those other functions unimplemented, and
crash.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>