Fixed race waiting for ARP response

We didn't start watching the condition until after sending the packet.
By then, it may have already sent the notification.
Use Lwt thread for ARP response

Using a Lwt_condition here is unnecessary complicated, and risks races
such as the one fixed in the previous commit.
bump working version to 2.0.3
Build Xen stubs separately, with CFLAGS from mirage-xen