Don't fail hard when we can't connect to the monitor

As of 1a50ba2c we fail to connect to the
monitor instead of getting an exit status != 0 from qemu itself.  This
breaks capabilities probing for the non QMP case.

src/qemu/qemu_capabilities.c

@@ -2345,8 +2345,10 @@ qemuCapsInitQMP(qemuCapsPtr caps,
         goto cleanup;
     }
 
-    if (!(mon = qemuMonitorOpen(NULL, &config, true, &callbacks)))
+    if (!(mon = qemuMonitorOpen(NULL, &config, true, &callbacks))) {
+        ret = 0;
         goto cleanup;
+    }
 
     qemuMonitorLock(mon);