CP-8866:Develop for Resource Statistic Feature on XenCenter

Based on the requirements draft and discussed solution, develop the prototype for Resource Statistic Feature part on XenCenter, that is, provide the export to Excel and CSV functionality with required data sets,by accessing the XenCenter cache.
Signed-off-by: Cheng Zhang <cheng.zhang@citrix.com>
The user experience when several pools are selected is a bit klunky: you have to fill in several Save As dialogs.

I suggest that we try writing all the pools to one .xls/.csv file, one after the other. If that is not possible, then let's just not support multiselect at all, and only enable the menu item when a single pool is selected.