The real Q35 machine puts the first USB controller set (EHCI+(UHCIx4))
on bus 0 slot 0x1D, and the 2nd USB controller set on bus 0 slot 0x1A,
so let's attempt to make the virtual machine match that for
controllers with auto-assigned addresses when possible.

Three test cases were added to assure that the proper addresses are
assigned - one with a single set of unaddressed USB controllers, one
with 3 (to grab both preferred slots plus one more), and one with the
order of the controller definitions reordered, to assure that the
auto-assignment isn't mixed up by order.