RFR: 6875: Add agent MXBean operation to return instrumented xml configuration

[Jessye Coleman-Shapiro]

This patch adds the MXBean operation `AgentControllerMXBean.retrieveEventProbes()` to the agent.  This function is the
counterpart of `AgentControllerMXBean.defineEventProbes()`.

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Commit messages:
 - create retrieveEventProbes() operation

Changes: https://git.openjdk.java.net/jmc/pull/99/files
 Webrev: https://webrevs.openjdk.java.net/jmc/99/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6875
  Stats: 141 lines in 6 files changed: 141 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/99.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/99/head:pull/99

PR: https://git.openjdk.java.net/jmc/pull/99