net/filter-rewriter.c: Make filter-rewriter support vnet_hdr_len

We add the vnet_hdr_support option for filter-rewriter, 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-rewriter,id=rew0,netdev=hn0,queue=all,vnet_hdr_support

We get the vnet_hdr_len from NetClientState that make us
parse net packet correctly.

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