tests : add check on the max nb of TDs that can be run in parralel

[hector-cao]

There is a limit on the number of TDs that can be run in parralel.                         
This limit can be due to several factors, but the most prevalent factor                    
is the number of keys the CPU can allocate to TDs.                                         
In fact, TDX takes advantage of an existing CPU feature called MK-TME                      
(Multi-key Total Memory Encryption) to encrypt the VM memory. It enables                   
the CPU to encrypt each TD’s memory with a unique Advanced Encryption Standard (AES) key.  
MK-TME offers a number of keys and this key space is partionned into 2 sets:               
Shared (VMM) and Private (TDX). The number of key in the Private space defines the         
maximum number of TDs we can run in parralel.                                              
                                                                                           
This test verifies that we can run TDs up to this limit and any new TD creation            
is refused by qemu in a nice way.

You can run this test with:

$ cd tests
$ sudo ./tdtest -k 'test_stress_max_guests'

[frenchwr]

LGTM!