Stash the packet length in a local variable before handing over
ownership of the packet to virtio_transport_recv_pkt() or
virtio_transport_free_pkt().

This patch solves the use-after-free since pkt is no longer guaranteed
to be alive.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>