tools/osbuild-image-info: append mountpoint to mount ID

The mount ID must be unique.  So far, we were using the device as the ID
for the mount because that was unique to each mount.  With btrfs
subvolumes however, the device and partition are the same for all, so we
need another way to differentiate.

tools/osbuild-image-info

@@ -2755,7 +2755,7 @@ def append_partitions(report, image):
 
                 # Finally mount
                 mnt_kwargs = {
-                    "name": part_device,
+                    "name": part_device + part_mountpoint,
                     "info": info,
                     # retrieves the associated Device Object
                     "device": devices_map[part_device],