Minor fixes for full-system timing memory.

Minor fixes for full-system timing memory.
Need to rewrite bus bridge to get any further.

src/dev/io_device.cc:
    Set packet dest on timing responses.
src/mem/bus.cc:
    Fix dest addr bounds check assertion.
    Add assertion to catch infinite loopbacks.
src/mem/physical.cc:
    Add comment.

--HG--
extra : convert_revision : 419b65a3a61e2d099884dbda117b338dffd80896