vfio/iommufd: invariant device name

cpr-transfer will use the device name as a key to find the value
of the device descriptor in new QEMU.  However, if the descriptor
number is specified by a command-line fd parameter, then
vfio_device_get_name creates a name that includes the fd number.
This causes a chicken-and-egg problem: new QEMU must know the fd
number to construct a name to find the fd number.

To fix, create an invariant name based on the id command-line parameter,
if id is defined.  The user will need to provide such an id to use CPR.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Link: https://lore.kernel.org/qemu-devel/1751493538-202042-10-git-send-email-steven.sistare@oracle.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

Author: Steve Sistare

hw/vfio/device.c

@@ -316,12 +316,17 @@ bool vfio_device_get_name(VFIODevice *vbasedev, Error **errp)
             error_setg(errp, "Use FD passing only with iommufd backend");
             return false;
         }
-        /*
-         * Give a name with fd so any function printing out vbasedev->name
-         * will not break.
-         */
         if (!vbasedev->name) {
-            vbasedev->name = g_strdup_printf("VFIO_FD%d", vbasedev->fd);
+
+            if (vbasedev->dev->id) {
+                vbasedev->name = g_strdup(vbasedev->dev->id);
+                return true;
+            } else {
+                /*
+                 * Assign a name so any function printing it will not break.
+                 */
+                vbasedev->name = g_strdup_printf("VFIO_FD%d", vbasedev->fd);
+            }
         }
     }