hyperv: scanning locking is using the bus mtx

[imp]

Summary

The scanning code uses Giant to coordinate its accesses to newbus as
well as to synchronize a little state within hyperv's vmbus. Switch to
the new bus_topo_* functions instead of referring to Giant explicitly.

Sponsored by: Netflix

[mav]

Looks good, but while at hyper-v I'd also do it for hv_pci_delete_device().

[kib]

Arguably, it is the strangest use of Giant from all code I ever see.