virtio: fix vring_init for 64 bits

This patch fixes a typo in vring_init().  This happens to work today in lguest
because the sizeof(struct vring_desc) is 16 and struct vring contains 3
pointers and an unsigned int so on 32-bit
sizeof(struct vring_desc) == sizeof(struct vring).  However, this is no longer
true on 64-bit where the bug is exposed.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>