virtio: export more headers to userspace

Rusty,

is there a reason why we dont export the virtio headers for
9p, balloon, console, pci, and virtio_ring? kvm uses make sync,
but I think it is still useful to heave these headers exported
as they might be useful for other userspace tools.

I dont export virtio.h, because it does not seem to have useful
information for userspace and it requires scatterlist.h which is
also not exported. See also my other mail about your "virtio:
change config to guest endian." patch.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>