qemu: auto-add pci-root controller for pc machine types

qemu: auto-add pci-root controller for pc machine types

<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.