CA-142313: XenCenter WLB Report "Run Reports" button should be disabled when popup.

Do not enable the "Run Report" button when none of the reports is selected.

Signed-off-by: Hui Zhang <hui.zhang@citrix.com>
Merge pull request #167 from xs-nanjing/CA-142313

CA-142313: XenCenter WLB Report "Run Reports" button should be disabled when popup.