SbNetwork: intf_defs collision when multiple instances of an SbDut have the same interface made external

[sgherbst]

Issue reported by @azaidy.

Problem is that the default name chosen is not necessarily unique. This would also apply if different SbDuts happened to have interfaces with the same name. A possible solution would be to create default names from the instance name and the interface name together. This should be guaranteed to be unique because instance names in a network must be unique, and interface names in a given instance must be unique.

On a separate note, name collisions should error out rather than clobbering silently.