<controller type='pci' index='0' model='pci-root'/>
is auto-added to pc* machine types.
Without this controller PCI bus 0 is not available and
no PCI addresses are assigned by default.
Since older libvirt supported PCI bus 0 even without
this controller, it is removed from the XML when migrating.
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
