vringh: fix copy direction of vringh_iov_push_kern()

We want to copy from iov to buf, so the direction was wrong.

Note: no real user for the helper, but it will be used by future
features.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>