um: vfio: Prevent duplicate device assignments

Ensure devices are assigned only once. Reject subsequent requests
for duplicate assignments.

Fixes: a0e2cb6a ("um: Add VFIO-based virtual PCI driver")
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250606124428.148164-4-tiwei.btw@antgroup.com


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