virtio-9p: record element after sanity checks

If the guest sends a malformed request, we end up with a dangling pointer
in V9fsVirtioState. This doesn't seem to cause any bug, but let's remove
this side effect anyway.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
