Simplify driver_ops for virtual interface add/remove

Simplify driver_ops for virtual interface add/remove

There is no absolute requirement for separating address allocation
into separate functions, so simplify the driver wrapper interface
to use just if_add and if_remove instead of adding the new
alloc_interface_addr() and release_interface_addr() functions.

if_add() can now indicate if the driver forced a different interface
name or address on the virtual interface.