RFR: 8268520: VirtualSpace::print_on() should be const

[Ioi Lam]

Please review this trivial patch. VirtualSpace::print_on() should be const so we can avoid the weird casting in epsilonHeap.cpp

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Commit messages:
 - 8268520: VirtualSpace::print_on() should be const

Changes: https://git.openjdk.java.net/jdk/pull/4448/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4448&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268520
  Stats: 6 lines in 3 files changed: 0 ins; 1 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4448.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4448/head:pull/4448

PR: https://git.openjdk.java.net/jdk/pull/4448