osbuild2: generalise copy stage option generation

Use entity based method `ForEachMountable` and `getDevices` to generate
all mounts and devices in a generic way, which then allows for mounts on
arbitrarily nested devices.

Update manifests and image info:
- New device names generated by `pathdot()` to avoid basename
  collisions.
- Some partitions are generated in a different order now which changes
  the order they appear in the manifest and their UUIDs.

Co-Authored-By: Christian Kellner <christian@kellner.me>

test/data/manifests/centos_9-aarch64-openstack-boot.json

@@ -1029,7 +1029,7 @@
                   "size": 1024000
                 }
               },
-              "efi": {
+              "boot.efi": {
                 "type": "org.osbuild.loopback",
                 "options": {
                   "filename": "disk.img",
@@ -1060,9 +1060,9 @@
                 "target": "/boot"
               },
               {
-                "name": "efi",
+                "name": "boot.efi",
                 "type": "org.osbuild.fat",
-                "source": "efi",
+                "source": "boot.efi",
                 "target": "/boot/efi"
               }
             ]
@@ -10255,7 +10255,7 @@
         "fstype": "xfs",
         "label": "root",
         "partuuid": "6264D520-3FB9-423F-8AB8-7A0A8E3D3562",
-        "size": 3559864320,
+        "size": 3559898624,
         "start": 735051776,
         "type": "0FC63DAF-8483-4772-8E79-3D69D8477DE4",
         "uuid": "6e4ff95f-f662-45ee-a82a-bdf44a2d0b75"