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.

tests/qemuxml2argvdata/qemuxml2argv-disk-usb.xml

@@ -27,6 +27,7 @@
     </disk>
     <controller type='usb' index='0'/>
     <controller type='ide' index='0'/>
+    <controller type='pci' index='0' model='pci-root'/>
     <memballoon model='virtio'/>
   </devices>
 </domain>