fix(xo-cli): better fallback logic for JSON-RPC transport

Logic:
- before: fallback on all network, HTTP or JSON-RPC formatting errors
- now: fallback only when response content-type is not `application/json`.

packages/xen-api/src/transports/json-rpc.js

@@ -19,21 +19,15 @@ export default ({ secureOptions, url, agent }) => {
       },
       method: 'POST',
       agent,
-    }).catch(error => {
-      console.warn('xen-api/transports/json-rpc', error)
-
-      throw new UnsupportedTransport()
     })
 
-    const text = await res.text()
-
-    let response
-    try {
-      response = parse(text)
-    } catch (error) {
+    // content-type is `text/xml` on old hosts where JSON-RPC is unsupported
+    if (res.headers['content-type'] !== 'application/json') {
       throw new UnsupportedTransport()
     }
 
+    const response = parse(await res.text())
+
     if (response.type === 'response') {
       return response.result
     }