ui/qmp-cmds: Improve two error messages

ui/qmp-cmds: Improve two error messages

set_password with "protocol": "vnc" supports only "connected": "keep".
Any other value is rejected with

    Invalid parameter 'connected'

Improve this to

    parameter 'connected' must be 'keep' when 'protocol' is 'vnc'

client_migrate_info requires "port" or "tls-port".  When both are
missing, it fails with

    Parameter 'port/tls-port' is missing

Improve this to

    parameter 'port' or 'tls-port' is required

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20231031111059.3407803-5-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>