sim, kvm: make KvmVM a System parameter

A KVM VM is typically a child of the System object already, but for
solving future issues with configuration graph resolution, the most
logical way to keep track of this object is for it to be an actual
parameter of the System object.

Change-Id: I965ded22203ff8667db9ca02de0042ff1c772220
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

configs/example/se.py

@@ -209,7 +209,7 @@ for cpu in system.cpu:
 
 if is_kvm_cpu(CPUClass) or is_kvm_cpu(FutureClass):
     if buildEnv['TARGET_ISA'] == 'x86':
-        system.vm = KvmVM()
+        system.kvm_vm = KvmVM()
         for process in multiprocesses:
             process.useArchPT = True
             process.kvmInSE = True