RFR: 6870: defineEventProbes is incorrect when probes occupy different classes

[Jessye Coleman-Shapiro]

Previously, when the agent MXBean operation `defineEventProbes(xmlDescription)` was called with an XML description only
containing probes from new classes, existing probes in other classes were not reverted. This patch fixes
`TransformRegistry.modify(xmlDescription)` so that this does not happen.

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Commit messages:
 - Make defineEventProbes work with probes in different classes

Changes: https://git.openjdk.java.net/jmc/pull/97/files
 Webrev: https://webrevs.openjdk.java.net/jmc/97/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6870
  Stats: 63 lines in 4 files changed: 15 ins; 20 del; 28 mod
  Patch: https://git.openjdk.java.net/jmc/pull/97.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/97/head:pull/97

PR: https://git.openjdk.java.net/jmc/pull/97