devices: add new vg_partition parameter

This is needed for bootc where all mounts need to be from the same
physical disk/loop so that bootupd works. The idea is that in the
manifest the new option `vg_partition` is added and the parent VG
is found via the partition number of the full image similar to
the `partnum` from osbuild#1501

So a manifest looks like:
```json
"devices": {
  "disk": {
    "type": "org.osbuild.loopback",
    "options": {
      "filename": "disk.raw",
      "partscan": true
    }
  },
  "rootlv": {
    "type": "org.osbuild.lvm2.lv",
    "parent": "disk",
    "options": {
      "volume": "rootlv",
      "vg_partnum": 4
    }
  }
}
```

devices/org.osbuild.lvm2.lv

@@ -44,6 +44,10 @@ SCHEMA = """
   "volume": {
     "type": "string",
     "description": "Logical volume to active"
+  },
+  "vg_partition": {
+    "type": "number",
+    "description": "Partition on the parent device where the LV's volume group is located"
   }
 }
 """
@@ -182,6 +186,9 @@ class LVService(devices.DeviceService):
 
         # Find the volume group that belongs to the device specified
         # via `parent`
+        part = options.get("vg_partition")
+        if part:
+            parent_path += f"p{part}"
         vg = self.volume_group_for_device(parent_path)
 
         # Activate the logical volume