Implementation of ExecuteVMOperationEvent

[zhengyu123]

ExecuteVMOperationEvent is an important event for observing and understanding VM latency, I believe it should be supported.

E.g.

jdk.ExecuteVMOperation {
  startTime = 09:20:31.803
  duration = 1.244 ms
  operation = "com.oracle.svm.core.genscavenge.GCImpl.CollectionVMOperation"
  safepoint = true
  blocking = true
  caller = "main" (javaThreadId = 1)
  safepointId = 2
  eventThread = "main" (javaThreadId = 1)
}

[christianhaeubl]

Thanks for contributing, I added a few comments.