<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.
Now we set the default disk driver name when parsing
the qemu command line too, hence all the test changes.
Assume format type is 'auto' when none is specified on
qemu command line.
