RFR: 6875: Add agent MXBean operation to return instrumented xml configuration [v2]

[Jessye Coleman-Shapiro]

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

Jessye Coleman-Shapiro has updated the pull request with a new target base due to a merge or a rebase. The pull request
now contains three commits:

 - Add javadoc for retrieveCurrentTransforms
 - Add test for invalid xml inputted
 - create retrieveEventProbes() operation

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Changes: https://git.openjdk.java.net/jmc/pull/99/files
 Webrev: https://webrevs.openjdk.java.net/jmc/99/webrev.01
  Stats: 157 lines in 7 files changed: 157 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