[RFC] exposing 'nodedev assigned to domain' info to users

Hi, This is something I've been giving a thought after working in Gitlab issue #72 and decided to run through the ML before hitting the code. We don't have an easy way to retrieve the domain that is using an specific hostdev. Let's say that I want to know which domain is using the PCI card pci_0000_01_00_2. 'nodedev-dumpxml' will return the hardware/driver capabilities of the device, such as IOMMU group, driver and so on, but will not inform which domain is using the hostdev, if any. 'nodedev-list' will simply list all nodedev names known to Libvirt, without outputting any other information. IIUC, the only existing way I can reliably tell whether a hostdev is being used by domain, aside from having to register the information by myself during domain definition of course, is via 'virsh dumpxml <domain>' each existing running domain and matching the nodedev name with the source.address element of the XML. When we consider SR-IOV devices that can have 28+ VFs each (and have lots of fun caveats, like Github #72 showed us), the capability of hot plug/unplug hostdevs freely, and lots of running domains, it is clear that we're putting a considerable pressure in the upper layers (OVirt, or a poor human admin) to keep track of the nodedevs each running domain is using. An info that we already have internally and can just expose it. I have a few ideas to make this happen: 1 - upgrade 'nodedev-list' to add an extra 'assigned to' column This is the more straightforward way of exposing the info. A simple 'nodedev-list' call can retrieve which domain is using which nodedev. To preserve the existing usage we can add an "--show-assigned-domains" option to control whether we will display this info. 2 - add an '<assigned_to>' element in nodedev XML definition I'm not a fan of exposing this in this particular XML because we would mix host/hw related attributes with domain info. But it would be easier to pull this off comparing to (1), so I'm mentioning it for the record. I would start by exposing the info for HOSTDEV_SUBSYS_TYPE_PCI hostdevs (--cap pci in nodedev-list). Thanks, DHB