Let user request a NBD server from GUI

@rafael I think it would be useful if people can request an NBD server to hold custom image. What do you think ?

/reviewed-on !552

software/kvm/instance-kvm-nbd-server-input-schema.json

+{
+  "type": "object",
+  "$schema": "http://json-schema.org/draft-04/schema",
+  "allOf": [
+    {
+      "$ref": "instance-kvm-nbd-server-input-schema.json#/"
+    },
+    {
+      "properties": {
+      }
+    }
+  ]
+}

software/kvm/instance-kvm-nbd-server-output-schema.json

+{
+  "name": "Output Parameters",
+  "properties": {
+    "nbd_url": {
+      "title": "NBD server URL",
+      "description": "URL to be used to boot another VM. Requires IPv6. IPv6 should be used as \"NBD hostname\" and port as \"NBD port\"",
+      "type": "string",
+      "format": "uri"
+    },
+    "upload_url": {
+      "title": "Upload URL",
+      "description": "URL used to upload your VM image.",
+      "type": "string",
+      "format": "uri"
+    },
+    "upload_key": {
+      "title": "Upload key",
+      "description": "Key used to upload your VM image.",
+      "type": "string",
+      "format": "uri"
+    }
+  }
+}

software/kvm/software.cfg.json

@@ -33,6 +33,13 @@
       "request": "instance-kvm-cluster-simplified-input-schema.json",
       "response": "instance-kvm-output-schema.json",
       "index": 3
+    },
+    "nbd": {
+      "title": "NBD Server",
+      "description": "Simple NBD server where you can upload one image. This is a one-time server. Create another server if you want to change the image.",
+      "request": "instance-kvm-nbd-server-input-schema.json",
+      "response": "instance-kvm-nbd-server-output-schema.json",
+      "index": 4
     }
   }
 }