Introduce vm_get_vcpu helper function.

Change-Id: I46aef6d240a74357b96b6677d1ec4c8386076f57
diff --git a/src/main.c b/src/main.c
index 6263c32..bd4c319 100644
--- a/src/main.c
+++ b/src/main.c
@@ -146,7 +146,7 @@
 		panic("mm_cpu_init failed");
 	}
 
-	vcpu = &vm_get(HF_PRIMARY_VM_ID)->vcpus[cpu_index(c)];
+	vcpu = vm_get_vcpu(vm_get(HF_PRIMARY_VM_ID), cpu_index(c));
 	vm = vcpu->vm;
 	vcpu->cpu = c;