RFR: 8296323: JVMTI can_support_virtual_threads not available for agents loaded into running VM [v7]

[Serguei Spitsyn]

> The can_support_virtual_thread was initially implemented as an onload capability.
> It is why this capability does not work for the agents loaded into running VM.
> The fix is to move it from `onload` to `always`capabilities list.
> 
> Testing:
> New test is added: VirtualStartThreadTest.
> TBD: mach5 jvmti, jdi and tier1-6 tests.

Serguei Spitsyn has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Merge branch 'master' into br19
   Merge
 - removed thread->vthread() != NULL from  JvmtiVirtualThreadEventMark constructor
 - minor update for unnamed threads in jvmti_common.h
 - fixed a trailing white space issue
 - extended VirtualThreadStartTest to support more configs; fixed issue in jvmtiExport.cpp
 - roll back unintended VirtualThread.java file update
 - simplified VirtualThreadStartTest
 - 8296323: JVMTI can_support_virtual_threads not available for agents loaded into running VM

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11246/files
  - new: https://git.openjdk.org/jdk/pull/11246/files/1a8f9810..7c659909

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11246&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11246&range=05-06

  Stats: 18790 lines in 156 files changed: 6558 ins; 2951 del; 9281 mod
  Patch: https://git.openjdk.org/jdk/pull/11246.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11246/head:pull/11246

PR: https://git.openjdk.org/jdk/pull/11246