Fixes for draining.

Fixes for draining.

src/cpu/simple/timing.cc:
    Update for changed return values.
src/python/m5/__init__.py:
    Loop in order to make sure all objects are really drained.  Objects may become undrained as other objects become drained (e.g. a bus-bridge has a packet, while a bus is empty, and the first drain() will cause the bus-bridge to give the packet to the bus).

    The only case we know every object is actually drained is if they all return immediately that they are drained.

--HG--
extra : convert_revision : 80057a1d6d30381bd0b67b23549bd202f447c5cb