vm.delete() must not remove unpluggable VDIs.

src/xapi.js

@@ -567,6 +567,11 @@ export default class Xapi extends XapiBase {
 
     if (deleteDisks) {
       await Promise.all(map(vm.$VBDs, vbd => {
+        // Do not delete unpluggable VDIs.
+        if (vbd.unpluggable) {
+          return
+        }
+
         try {
           return this._deleteVdi(vbd.$VDI).catch(noop)
         } catch (_) {}