Use indexing in preference to pointer arithmetic.

Change-Id: Ia90714e95af1e545f626bd5b6c99e40622e37286
diff --git a/src/main.c b/src/main.c
index e6ba2f1..7326cb0 100644
--- a/src/main.c
+++ b/src/main.c
@@ -128,5 +128,5 @@
 
 	vm_set_current(&primary_vm);
 
-	return primary_vm.vcpus + cpu_index(c);
+	return &primary_vm.vcpus[cpu_index(c)];
 }