Throw an exception if an @EJB can be resolved to multiple ambiguous EJBs

When use (@EJB(name = "XYZBean.EJB_NAME"), it specified the name under which the reference is bound in java:comp/env,
With different JDK e.g. Oracle JDK7 and IBM JDK6/7, that could cause problem if more than one classes implement the interface and this @EJB annotation could be resolved to multiple EJBs.
But they're still all valid choice during the bean injection. So we'd better add an explicit log warning of this ambiguity for user.