um: Add VFIO-based virtual PCI driver

Implement a new virtual PCI driver based on the VFIO framework.
This driver allows users to pass through PCI devices to UML via
VFIO. Currently, only MSI-X capable devices are supported, and
it is assumed that drivers will use MSI-X.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250413154421.517878-1-tiwei.btw@antgroup.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>