newbus: add bus_topo_assert

[imp]

Summary

Add bus_topo_assert() and implmement it as GIANT_REQUIRED for the
moment. This will allow us to change more easily to a newbus-specific
lock int he future.

Sponsored by: Netflix

Test Plan

Note: I think the giant manipulation in hyperv/vmbus is related to newbus, but there's more to it that I've not read through the code.
I've tried to add reviews for all the maintainers of this code, even though the change is, at this moment, syntactic sugar only.

[imp]

Add topo assert in mii_attach