kvm tools: Introduce uip_rx() for uip

kvm tools: Introduce uip_rx() for uip

This patch implement rx interface for uip. uip_rx() can be called in
virtio_net_rx_thread().

It is a consumer of the ethernet used buffer. It sleeps until there is
used buffer avaiable and copy ethernet data into virtio iov buffers
which provided by virtio_net_rx_thread().

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