config: Add a 'kvm' CPU alias

Add a CPU alias, 'kvm', for the first available KVM-accelerated CPU
model.

configs/common/CpuConfig.py

@@ -52,6 +52,7 @@ _cpu_aliases_all = [
     ("atomic", "AtomicSimpleCPU"),
     ("inorder", "InOrderCPU"),
     ("detailed", "DerivO3CPU"),
+    ("kvm", ("ArmKvmCPU", "X86KvmCPU")),
     ]
 
 # Filtered list of aliases. Only aliases for existing CPUs exist in