Execute "ethtool -L eth0 combined 0" in guest, if multiqueue
is enabled, virtnet_send_command() will return -EINVAL error,
there is a validation in QEMU.

But if multiqueue is disabled, virtnet_set_queues() will just
return zero (success). We should return error for this situation.
Signed-off-by: NAmos Kong <akong@redhat.com>
Acked-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>