OvmfPkg: SEC/SMM: Use 256 IDT Descriptors

This follows the update in UefiCpuPkg to use
256 IDT descriptors in SEC and SMM. This is
done to:
- Have a defined action when exceptions are
taken (use the common exception handler)
- Allow processing the stack cookie exception
gracefully instead of generating a #GP fault.

Signed-off-by: Oliver Smith-Denny <osde@microsoft.com>