config: Improve ruby simobject names

This patch attaches ruby objects to the system before the topology is
created so that their simobject names read their meaningful variable
names instead of their topology name.

configs/example/memtest-ruby.py

@@ -111,7 +111,7 @@ system = System(cpu = cpus,
                 funcmem = PhysicalMemory(),
                 physmem = PhysicalMemory())
 
-system.ruby = Ruby.create_system(options, system.physmem)
+system.ruby = Ruby.create_system(options, system)
 
 assert(len(cpus) == len(system.ruby.cpu_ruby_ports))