fix #1691: use asynchronous API call for HD create

It should use an asynchronous API call to create an image,
because some storages may need longer to alloc an image.

If a storage needs more than 30 sec
the proxy will timeout and we have an orphan image.

Also, it is possible to increase the timeout for some
storages if an asynchronous call is used

www/manager6/qemu/HDEdit.js

@@ -364,6 +364,8 @@ Ext.define('PVE.qemu.HDEdit', {
 
     isAdd: true,
 
+    backgroundDelay: 5,
+
     initComponent : function() {
 	var me = this;