vhost: reserve some space in structures

vhost: reserve some space in structures

So that we will not break ABI in future extension by adding few more
fields.

Struct vhost_virtqueue is reserved with 16 qwords (the later vhost-live
migration support would at least consume 3 of them), and struct virtio_net
is reserved with a bit more, 64 qwords, as there is only one instance for
a virtio nic instance.

Note that both reservation are not placed at the end of the struct, but
instead before the last field, since both the last field at the two struct
take a lot spaces. Putting the reservation after it would divide those
reserved fields to another cacheline. (we might need fix them in future, btw)

Suggested-by: Panu Matilainen <pmatilai@redhat.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>