Add PREALLOCATE mode to mt_tbs.py.

This goes through each trace file once at the beginning of the run to
identify all unique virtual pages and create a virtual to physical page
mapping. Other threads that use the same trace file will then use that
set of virtual pages to allocate their own physical pages before execution
starts.

Other minor changes:
* Updated ini/32mb_loops.yaml to not use base_addresses, which are imcompatible
  with the virtual address mode.
* Updated HybridSim's mmio() method to not print out a message on cerr for every
  prefetch instruction received (which overwhelms the output).