qmp-commands: move 'block_passwd' doc to schema

Returns: Nothing on success

         If "device" does not exist or cannot be determined, DeviceNotFound

block_passwd

------------

Set the password of encrypted block devices.

Arguments:

- "device": device name (json-string)

- "node-name": name in the block driver state graph (json-string)

- "password": password (json-string)

Example:

-> { "execute": "block_passwd", "arguments": { "device": "ide0-hd0",

                                               "password": "12345" } }

<- { "return": {} }

block_set_io_throttle

------------

#         occur if an invalid password is specified.

#

# Since: 0.14.0

#

# Example:

#

# -> { "execute": "block_passwd", "arguments": { "device": "ide0-hd0",

#                                                "password": "12345" } }

# <- { "return": {} }

#

##

{ 'command': 'block_passwd', 'data': {'*device': 'str',

                                      '*node-name': 'str', 'password': 'str'} }