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

[Joshua Matsuoka]

On Fri, 24 Jul 2020 22:42:37 GMT, Jessye Coleman-Shapiro <jescolem at openjdk.org> wrote:

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

Hi Jessye,

Changes look good to me!

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