qmp: Print descriptions of object properties

Add a new "description" field to DevicePropertyInfo.
The descriptions can serve as documentation in the code,
and they can be used to provide better help. For example:

$./qemu-system-x86_64 -device virtio-blk-pci,?

Before this patch:

virtio-blk-pci.iothread=link<iothread>
virtio-blk-pci.x-data-plane=bool
virtio-blk-pci.scsi=bool
virtio-blk-pci.config-wce=bool
virtio-blk-pci.serial=str
virtio-blk-pci.secs=uint32
virtio-blk-pci.heads=uint32
virtio-blk-pci.cyls=uint32
virtio-blk-pci.discard_granularity=uint32
virtio-blk-pci.bootindex=int32
virtio-blk-pci.opt_io_size=uint32
virtio-blk-pci.min_io_size=uint16
virtio-blk-pci.physical_block_size=uint16
virtio-blk-pci.logical_block_size=uint16
virtio-blk-pci.drive=str
virtio-blk-pci.virtio-backend=child<virtio-blk-device>
virtio-blk-pci.command_serr_enable=on/off
virtio-blk-pci.multifunction=on/off
virtio-blk-pci.rombar=uint32
virtio-blk-pci.romfile=str
virtio-blk-pci.addr=pci-devfn
virtio-blk-pci.event_idx=on/off
virtio-blk-pci.indirect_desc=on/off
virtio-blk-pci.vectors=uint32
virtio-blk-pci.ioeventfd=on/off
virtio-blk-pci.class=uint32

After:

virtio-blk-pci.iothread=link<iothread>
virtio-blk-pci.x-data-plane=bool (on/off)
virtio-blk-pci.scsi=bool (on/off)
virtio-blk-pci.config-wce=bool (on/off)
virtio-blk-pci.serial=str
virtio-blk-pci.secs=uint32
virtio-blk-pci.heads=uint32
virtio-blk-pci.cyls=uint32
virtio-blk-pci.discard_granularity=uint32
virtio-blk-pci.bootindex=int32
virtio-blk-pci.opt_io_size=uint32
virtio-blk-pci.min_io_size=uint16
virtio-blk-pci.physical_block_size=uint16 (A power of two between 512 and 32768)
virtio-blk-pci.logical_block_size=uint16 (A power of two between 512 and 32768)
virtio-blk-pci.drive=str (ID of a drive to use as a backend)
virtio-blk-pci.virtio-backend=child<virtio-blk-device>
virtio-blk-pci.command_serr_enable=bool (on/off)
virtio-blk-pci.multifunction=bool (on/off)
virtio-blk-pci.rombar=uint32
virtio-blk-pci.romfile=str
virtio-blk-pci.addr=int32 (Slot and optional function number, example: 06.0 or 06)
virtio-blk-pci.event_idx=bool (on/off)
virtio-blk-pci.indirect_desc=bool (on/off)
virtio-blk-pci.vectors=uint32
virtio-blk-pci.ioeventfd=bool (on/off)
virtio-blk-pci.class=uint32

Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>

qapi-schema.json

@@ -1615,11 +1615,13 @@
 #
 # @name: the name of the property
 # @type: the typename of the property
+# @description: #optional if specified, the description of the property.
+#               (since 2.2)
 #
 # Since: 1.2
 ##
 { 'type': 'DevicePropertyInfo',
-  'data': { 'name': 'str', 'type': 'str' } }
+  'data': { 'name': 'str', 'type': 'str', '*description': 'str' } }
 
 ##
 # @device-list-properties:

qdev-monitor.c

@@ -213,9 +213,14 @@ int qdev_device_help(QemuOpts *opts)
     }
 
     for (prop = prop_list; prop; prop = prop->next) {
-        error_printf("%s.%s=%s\n", driver,
+        error_printf("%s.%s=%s", driver,
                      prop->value->name,
                      prop->value->type);
+        if (prop->value->has_description) {
+            error_printf(" (%s)\n", prop->value->description);
+        } else {
+            error_printf("\n");
+        }
     }
 
     qapi_free_DevicePropertyInfoList(prop_list);

qmp.c

@@ -442,7 +442,8 @@ ObjectTypeInfoList *qmp_qom_list_types(bool has_implements,
  */
 static DevicePropertyInfo *make_device_property_info(ObjectClass *klass,
                                                      const char *name,
-                                                     const char *default_type)
+                                                     const char *default_type,
+                                                     const char *description)
 {
     DevicePropertyInfo *info;
     Property *prop;
@@ -465,7 +466,9 @@ static DevicePropertyInfo *make_device_property_info(ObjectClass *klass,
 
             info = g_malloc0(sizeof(*info));
             info->name = g_strdup(prop->name);
-            info->type = g_strdup(prop->info->legacy_name ?: prop->info->name);
+            info->type = g_strdup(prop->info->name);
+            info->has_description = !!prop->info->description;
+            info->description = g_strdup(prop->info->description);
             return info;
         }
         klass = object_class_get_parent(klass);
@@ -475,6 +478,9 @@ static DevicePropertyInfo *make_device_property_info(ObjectClass *klass,
     info = g_malloc0(sizeof(*info));
     info->name = g_strdup(name);
     info->type = g_strdup(default_type);
+    info->has_description = !!description;
+    info->description = g_strdup(description);
+
     return info;
 }
 
@@ -521,7 +527,8 @@ DevicePropertyInfoList *qmp_device_list_properties(const char *typename,
             continue;
         }
 
-        info = make_device_property_info(klass, prop->name, prop->type);
+        info = make_device_property_info(klass, prop->name, prop->type,
+                                         prop->description);
         if (!info) {
             continue;
         }