Fix logic for assigning PCI addresses to USB2 companion controllers

Fix logic for assigning PCI addresses to USB2 companion controllers

Currently each USB2 companion controller gets put on a separate
PCI slot. Not only is this wasteful of PCI slots, but it is not
in compliance with the spec for USB2 controllers. The master
echi1 and all companion controllers should be in the same slot,
with echi1 in function 7, and uhci1-3 in functions 0-2 respectively.

* src/qemu/qemu_command.c: Special case handling of USB2 controllers
  to apply correct pci slot assignment
* tests/qemuxml2argvdata/qemuxml2argv-usb-ich9-ehci-addr.args,
  tests/qemuxml2argvdata/qemuxml2argv-usb-ich9-ehci-addr.xml: Expand
  test to cover automatic slot assignment