Re: [libvirt] [PATCH v2 1/4] domain_conf: allow address type='none' to unassign PCI hostdevs

Today, to use a PCI hostdev "A" in a domain, all PCI devices that belongs to the same IOMMU group must also be declared in the domain XML, meaning that all IOMMU devices are detached from the host and all of them are visible to the guest. The result is that the guest will have access to all devices, but this is not necessarily what the administrator wanted. If only the hostdev "A" was intended for guest usage, but hostdevs "B" and "C" happened to be in the same IOMMU group of "A", the guest will gain access to all 3 devices. This makes the administrator rely on alternative solutions, such as use all hostdevs with un-managed mode and detached all the IOMMU before the guest starts. If use un-managed mode is not an option, the only alternative left is an ACS patch to deny guest access to "B" and "C". Discussions made in [1] and [2] led to the approach implemented here. This patch builds upon the already existing VIR_DOMAIN_DEF_FORMAT_ADDRESS_NONE format to use it as a indication of whether a hostdev will be owned by a domain, but not visible to the guest OS. This allows the administrator to declare all the IOMMU while also choosing which hostdevs will be usable by the guest. This new mechanic applies to all PCI hostdevs, regardless of whether they are a PCI multifunction hostdev or not. Using <address type='none'/> in any case other than a PCI hostdev will result in error. The use of a new 'unassigned' flag in virDomainHostdevDef makes it easier to distinguish between the case we're handling here versus the case in which a PCI hostdev has no <address> element. Both are interpreted as VIR_DOMAIN_DEF_FORMAT_ADDRESS_NONE in the existing code, thus reducing the logic to a flag makes it easier to handle this new use case. In the next patch we'll use the 'unassigned' flag to filter the hostdevs being initialized in the QEMU command line.