VirtualThreadStart events have to be similar to ThreadStart events which are not thread-filtered.

Also need a couple of unification tweaks:
  - to align JvmtiExport::post_thread_end() with JvmtiExport::post_vthread_end() to have a unified check for the JVMTI_PHASE_PRIMORDIAL.
  - to rename the local variable cur_thread as thread to follow the common pattern in JvmtiExport::post_vthread_start() and JvmtiExport::post_vthread_end()