Backends could provide a packet whose length is greater than buffer
size. Check for this and truncate the packet to avoid rx buffer
overflow in this case.

Cc: Prasad J Pandit <pjp@fedoraproject.org>
Cc: qemu-stable@nongnu.org
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>