According to ark.intel.com, the T5900 series processors do not support VT-x. Not sure why your bios indicates that you have it. Perhaps it supports some VT features but not all the VT features that HAXM requires.
Here are some trouble-shooting tips for installing HAXM. http://software.intel.com/en-us/blogs/2014/03/14/troubleshooting-intel-haxm