On VLAN subinterface creation, we can not set the tag of the interface.
Using the interface tag in the openstack ml2 plugin, an atomic operation for the creation of the subif AND the tagging is highly desired.

This implies an API modification:
define create_vlan_subif
{
u32 client_index;
u32 context;
u32 sw_if_index;
u32 vlan_id;
u8 tag[64];
};