net/filter-mirror.c: Make filter mirror support vnet support.

net/filter-mirror.c: Make filter mirror support vnet support.

We add the vnet_hdr_support option for filter-mirror, default is disabled.
If you use virtio-net-pci or other driver needs vnet_hdr, please enable it.
You can use it for example:
-object filter-mirror,id=m0,netdev=hn0,queue=tx,outdev=mirror0,vnet_hdr_support

If it has vnet_hdr_support flag, we will change the sending packet format from
struct {int size; const uint8_t buf[];} to {int size; int vnet_hdr_len; const uint8_t buf[];}.
make other module(like colo-compare) know how to parse net packet correctly.

Signed-off-by: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>