RFR: 8296324: JVMTI GetStackTrace truncates vthread stack trace for agents loaded into running VM [v2]

[Serguei Spitsyn]

> The `VirtualThread` static field `notifyJvmtiEvents` is not set correctly in cases JVMTI agents are loaded into running VM. It is because an extra call to java_lang_VirtualThread::init_static_notify_jvmti_events() is needed.
> This function is called once at the VM initialization, so this extra call is not necessary for agent loaded at startup.
> 
> Testing:
> New test is added: `test/hotspot/jtreg/serviceability/jvmti/vthread/VirtualStackTraceTest`
> This test is failed without fix and passed with it.
> TBD: run all JVMTI and JDI test in mach5.

Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:

  add @requires vm.jvmti to VirtualStackTraceTest

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11204/files
  - new: https://git.openjdk.org/jdk/pull/11204/files/b995eb44..5a15a6da

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11204&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11204&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11204.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11204/head:pull/11204

PR: https://git.openjdk.org/jdk/pull/11204