tools/virtio: separate headers more.

This makes them a bit more like the kernel headers, so we can include more
real kernel headers in our tests.

In addition this means that we don't break tools/virtio with the next
patch.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/tools/virtio/linux/uio.h b/tools/virtio/linux/uio.h
new file mode 100644
index 0000000..ecbdf92
--- /dev/null
+++ b/tools/virtio/linux/uio.h
@@ -0,0 +1 @@
+#include "../../../include/linux/uio.h"