[NET] front: Discard packets in tx ring rather than attempting retransmit

[NET] front: Discard packets in tx ring rather than attempting retransmit
when reconnecting to backend driver (e.g., after save/restore or migrate).

Two main reasons for this:
 1. The retransmit code is broken for fragmented packets. It would need
    a rewrite to cope with the new scatter-gather format.
 2. We will drop packets anyway, in both directions (e.g., takes some
    time for received packets to be redirected to new virtual interface;
    also further transmitted packets from the network stack are dropped
    after we call netif_carrier_off(), so even if we retransmite what's
    already in the ring it is likely that some subsequent packets will
    already be lost).

If this causes downtimes that are too long (particularly for live
relocation) then the whole strategy for buffering packets while the
frontend-backend connection is severed needs to be considered.

Signed-off-by: Keir Fraser <keir@xensource.com>