kvm tools: Add IPV4 support for uip

kvm tools: Add IPV4 support for uip

- Introduce struct uip_ip to present IP package

- Add a helper uip_ip_len() to return totoal length of a IP package

- Add a helper uip_ip_hdrlen() to return the IP header length

- Currently, uip does not support IP options
  Drop IP package if IP header length is not 20 bytes which means it
  contains IP options.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>