Introduce vm_get_vcpu helper function.

Change-Id: I46aef6d240a74357b96b6677d1ec4c8386076f57
diff --git a/inc/hf/vm.h b/inc/hf/vm.h
index a6bda8b..c54c5f0 100644
--- a/inc/hf/vm.h
+++ b/inc/hf/vm.h
@@ -97,3 +97,4 @@
 struct vm *vm_get(uint32_t id);
 struct vm_locked vm_lock(struct vm *vm);
 void vm_unlock(struct vm_locked *locked);
+struct vcpu *vm_get_vcpu(struct vm *vm, uint32_t vcpu_index);