osbuild: introduce device host service

A new host service that provides device functionality to stages.
Since stages run in a container and are restricted from creating
device nodes, all device handling is done in the main osbuild
process. Currently this is done with the help of APIs and RPC,
e.g. `LoopServer`. Device host services on the other hand allow
declaring devices in the manifest itself and then osbuild will
prepare all devices before running the stage. One desired effect
is that it makes device handling transparent to the stages, e.g.
they don't have to know about loopback devices, LVM or LUKS.
Another result is that specific device handling is now modular
like Inputs and Source are and thus moved out of osbuild itself.

schemas/osbuild2.json

@@ -16,6 +16,26 @@
 
   "definitions": {
 
+    "devices": {
+      "title": "Collection of devices for a stage",
+      "additionalProperties": {
+        "$ref": "#/definitions/device"
+      }
+    },
+
+    "device": {
+      "title": "Device for a stage",
+      "additionalProperties": false,
+      "required": ["type", "options"],
+      "properties": {
+        "type": { "type": "string" },
+        "options": {
+          "type": "object",
+          "additionalProperties": true
+        }
+      }
+    },
+
     "inputs": {
       "title": "Collection of inputs for a stage",
       "additionalProperties": false,
@@ -102,6 +122,7 @@
       "additionalProperties": false,
       "properties": {
         "type": { "type": "string" },
+        "devices": { "$ref": "#/definitions/devices" },
         "inputs": {"$ref": "#/definitions/inputs" },
         "options": {
           "type": "object",