stages/dmverity: make device objects more generic

We need to pass loopback devices for these properties, but the schema
says that there will be a `path` property, so osbuild complains.

osbuild is right of course, but this definitely *did* work in an earlier
version, so something changed. Ideally, we'd narrow down here what
happened exactly, but at the same time this approach of just making the
property more generic matches what's done in e.g. the `zipl.inst` stage
where we also use a loopback device.

For reference, this is where we use this stage:

https://github.com/coreos/coreos-assembler/blob/ba45b296ec11734bafcae7728915016f17137a3d/src/osbuild-manifests/platform.qemu.ipp.yaml#L100-L119

stages/org.osbuild.dmverity.meta.json

@@ -30,27 +30,11 @@
       "properties": {
         "data_device": {
           "type": "object",
-          "additionalProperties": false,
-          "required": [
-            "path"
-          ],
-          "properties": {
-            "path": {
-              "type": "string"
-            }
-          }
+          "additionalProperties": true
         },
         "hash_device": {
           "type": "object",
-          "additionalProperties": false,
-          "required": [
-            "path"
-          ],
-          "properties": {
-            "path": {
-              "type": "string"
-            }
-          }
+          "additionalProperties": true
         }
       }
     }