config: Do not instantiate membus when using ruby

This patch moves the instantiation of system.membus in se.py to the area of
code where classic memory system has been dealt with. Ruby does not require
this bus and hence it should not be instantiated.

configs/example/se.py

@@ -159,7 +159,7 @@ if options.smt and options.num_cpus > 1:
 np = options.num_cpus
 system = System(cpu = [CPUClass(cpu_id=i) for i in xrange(np)],
                 physmem = MemClass(range=AddrRange("512MB")),
-                membus = CoherentBus(), mem_mode = test_mem_mode)
+                mem_mode = test_mem_mode)
 
 # Sanity check
 if options.fastmem:
@@ -225,6 +225,7 @@ if options.ruby:
             system.cpu[i].itb.walker.port = ruby_port.slave
             system.cpu[i].dtb.walker.port = ruby_port.slave
 else:
+    system.membus = CoherentBus()
     system.system_port = system.membus.slave
     system.physmem.port = system.membus.master
     CacheConfig.config_cache(options, system)