The X5450 does not have the Extended Page Tables (EPT) feature. The EPT feature is available on Nehalem CPUs (circa 2009) and later.
Without EPT, you can only run 32-bit guest VMs inside a guest hypervisor. There is no software configuration to remedy your situation. You need to have CPU that has EPT (for Intel) or RVI (for AMD);
Have a read of this document Running Nested VMs