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: NRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>