FIX: resolve virtual page use with widgets.
FIX: resolve virtual page use with widgets.

When using a VirtualPage mapping to a BlogTree, widgets will bypass the instanceof BlogTree check and instead fail to find the current page.
scott1702/silverstripe-blog,Internetrix/silverstripe-blog,MichaelJJames/silverstripe-blog,souldigital/silverstripe-blog,souldigital/silverstripe-blog,scott1702/silverstripe-blog,tractorcow/silverstripe-blog,MichaelJJames/silverstripe-blog,steiha/silverstripe-blog,steiha/silverstripe-blog,tractorcow/silverstripe-blog,Internetrix/silverstripe-blog