Align the l3_hdr member of NetTxPkt by defining it as a union of
ip_header, ip6_header, and an array of octets.
Fixes: e263cd49c7 ("Packet abstraction for VMWARE network devices")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1544
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>