virtio: fix up VQ checks When migration triggers before a VQ is initialized, base pa is 0 and last_used_index must be 0 too: we don't have a ring to compare to. Reported-by: Juan Quintela <quintela@redhat.com> Tested-by: Juan Quintela <quintela@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>