ruby: add default ctor for MachineID type

not all uses of MachineID initialize its fields, so here we add a default
ctor.

src/mem/ruby/common/MachineID.hh

@@ -37,6 +37,10 @@
 
 struct MachineID
 {
+    MachineID() : type(MachineType_NULL), num(0) { }
+    MachineID(MachineType mach_type, NodeID node_id)
+        : type(mach_type), num(node_id) { }
+
     MachineType type;
     //! range: 0 ... number of this machine's components in system - 1
     NodeID num;