virtio_ring: document alignment requirements

Host needs to know vring element alignment requirements:
simply doing alignof on structures doesn't work reliably: on some
platforms gcc has alignof(uint32_t) == 2.

Add macros for alignment as specified in virtio 1.0 cs01,
export them to userspace as well.

Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>