RFR: 6875: Add agent MXBean operation to return instrumented xml configuration [v3]

[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 incrementally with one additional commit since the last revision:

  Add javadoc and test for retrieving an invalid configuration

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Changes:
  - all: https://git.openjdk.java.net/jmc/pull/99/files
  - new: https://git.openjdk.java.net/jmc/pull/99/files/16ea4d3d..80b5757a

Webrevs:
 - full: https://webrevs.openjdk.java.net/jmc/99/webrev.02
 - incr: https://webrevs.openjdk.java.net/jmc/99/webrev.01-02

  Stats: 21 lines in 2 files changed: 18 ins; 0 del; 3 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