kvm tools: Introduce ethernet frame buffer system for uip

kvm tools: Introduce ethernet frame buffer system for uip

- uip_buf_get_free()
  Get a free buffer from buffer pool, sleep if there is no free buffer.

- uip_buf_get_used()
  Get a used buffer from buffer pool, sleep if there is no used buffer.

- uip_buf_set_free()
  Set a buffer as free, so it can be reused by the buffer producer.

- uip_buf_set_used()
  Set a buffer as used, uip rx code will inject the ethernet frame in
  this buffer into guest.

- uip_buf_clone()
  Get a free buffer, and clone data into it.

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