qdev: Add help for property value

When called with property value "?",
a help text will be printed (instead of an error message).

This is useful for command lines like
    	qemu -device e1000,mac=?
and is already standard for other command line options.

A better help text could be provided by extending
the Property structure with a desc field.
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/qdev-properties.c

@@ -565,8 +565,13 @@ int qdev_prop_parse(DeviceState *dev, const char *name, const char *value)
         return -1;
     }
     if (prop->info->parse(dev, prop, value) != 0) {
-        fprintf(stderr, "property \"%s.%s\": failed to parse \"%s\"\n",
-                dev->info->name, name, value);
+        if (strcmp(value, "?") != 0) {
+            fprintf(stderr, "property \"%s.%s\": failed to parse \"%s\"\n",
+                    dev->info->name, name, value);
+        } else {
+            fprintf(stderr, "%s.%s=%s\n",
+                    dev->info->name, name, prop->info->name);
+        }
         return -1;
     }
     return 0;