Reconnect

This patch set implements a simple "reconnect" feature on the snabbswitch side only.
It does 2 things:
 - ensures that the buffers are used from the avail ring and returned to the used ring in ascending order
 - restires the txavai/rxavail from the used.idx of the ring

This works under no or light load (ping).

Under heavy load one needs to restart the interface in the virtual machine, for example:
ifconfig eth0 down && ifconfig eth0 up

NOTE: This feature is not tested enough for production quality. It relies on buffers being used in a particular order.