net: pktgen: disable xmit_clone on virtual devices

Trying to use burst capability (aka xmit_more) on a virtual device
like bonding is not supported.

For example, skb might be queued multiple times on a qdisc, with
various list corruptions.

Fixes: 38b2cf29

 ("net: pktgen: packet bursting via skb->xmit_more")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>