net: Add a 'do_not_pad" to NetClientState

This adds a flag in NetClientState, so that a net client can tell
its peer that the packets do not need to be padded to the minimum
size of an Ethernet frame (60 bytes) before sending to it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

include/net/net.h

@@ -103,6 +103,7 @@ struct NetClientState {
     int vring_enable;
     int vnet_hdr_len;
     bool is_netdev;
+    bool do_not_pad; /* do not pad to the minimum ethernet frame length */
     QTAILQ_HEAD(, NetFilterState) filters;
 };