Bus: Make the default bus width 8 bytes instead of 64

This patch changes the default bus width to a more sensible 8 bytes
(64 bits), which is in line with most on-chip buses. Although there
are cases where a wider or narrower bus is useful, the 8 bytes is a
good compromise to serve as the default.

This patch changes essentially all statistics, and will be bundled
with the outstanding changes to the bus.

src/mem/Bus.py

@@ -49,7 +49,7 @@ class BaseBus(MemObject):
     master = VectorMasterPort("vector port for connecting slaves")
     clock = Param.Clock("1GHz", "bus clock speed")
     header_cycles = Param.Int(1, "cycles of overhead per transaction")
-    width = Param.Int(64, "bus width (bytes)")
+    width = Param.Int(8, "bus width (bytes)")
     block_size = Param.Int(64, "The default block size if not set by " \
                                "any connected module")