details: Don't log exception if VM has no arch

Like xen Domain-0. Exception was harmless but not really useful

src/virtManager/details.py

@@ -2702,11 +2702,13 @@ class vmmDetails(vmmGObjectUI):
 
         capscpu = None
         try:
-            cpu_values = caps.get_cpu_values(self.vm.get_arch())
+            arch = self.vm.get_arch()
-            for c in cpu_values.cpus:
+            if arch:
-                if model and c.model == model:
+                cpu_values = caps.get_cpu_values(arch)
-                    capscpu = c
+                for c in cpu_values.cpus:
-                    break
+                    if model and c.model == model:
+                        capscpu = c
+                        break
         except:
             pass