<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.
Like "rawio", "sgio" is only allowed for block disk of device
type "lun".
It doesn't default disk->sgio to "filtered" when parsing, as
it won't be able to distinguish explicitly requested "filtered"
and a default "filtered" in driver then. We have to error out for
explicit request when the kernel doesn't support the new sysfs
knob "unpriv_sgio", however, for defaulted "filtered", we can
just ignore it if the kernel doesn't support "unpriv_sgio".
