VirtualSpace::print_on() should be const

VirtualSpace::print_on() and VirtualSpace::print() are not declared as const, leading to weird code like this in epsilonHeap.cpp

void EpsilonHeap::print_on(outputStream *st) const {
  st->print_cr("Epsilon Heap");

  // Cast away constness:
  ((VirtualSpace)_virtual_space).print_on(st);